Old Master Cylinder
Below are subtopics that follow related to the Master Cylinder -
Dave wrote –
I replaced the master cylinder in my '65 Beetle, but that was so many years ago I've almost forgotten what it looked like! :-) Volkswagen brakes are easy to work on.
Rob responded –
The only difference these days is the dual circuits, meaning there are two outlet pipes on the master cylinder, rather than just one. The dual master cylinder can be a little difficult to get rid of all the internal air after a rebuild or replacement, but with two people, you can loosen each output line (one at a time of course) whilst the other person applies the brake, then tighten it up before the pedal is released in effect using the tightening collar to pre-bleed the master cylinder (both chambers) before bleeding the wheel cylinders. They don't need much loosening (1/2 turn or so) to 'leak' under pressure. Makes a bit of mess 'cause it's hard to catch the 'leaking' fluid, but does the job.
There was originally a different disc master cylinder to the drum master cylinder (pressure bleed holes instead of valves to allow for the actuating rod to return with the released pedal), but I believe there is now a universal master cylinder which works with either, so the replacement (if you need one) may look just a little different to the original.
Old Master Cylinder
Dave -
I need to take a close look at the brakes; once in a while (not often) the pedal goes almost to the floor before we get any braking action. Kinda scary. I'm wondering about the condition of the master cylinder.
Rob –
Have you checked the fluid level in the brake reservoir? Left hand side inside the hood. It should be about 3/4 full. If nearly empty you might be getting air into the lines.
Pull up your floor covering around the pedals and look for any wet spots around the nose of the master cylinder (it has a push rod going from the pedal through a rubber boot into the cylinder, which bolts up to the body from the outside).
Master cylinders aren't too expensive to replace. Sometimes you can get repair kits (piston seals etc.) and these work for a while, but in worn cylinders they won't last all that long. I've got Brazilllian replacement rear slave cylinders on mine, and they work just fine -- about 5 years old, so no problems with the quality. The master cylinder might be OK from there too.
Dave –
I checked the fluid reservoir and found that it is NOT low. The fluid level is right at the seam between the top and the body of the reservoir, which I think is right where it is supposed to be. From what you are saying it sounds like our master cylinder has gone south.
Dave -
I put a thin piece of cardboard down into the reservoir as a dipstick (not realizing at the time that there are two chambers), and it came up showing the reservoir to be full, right at the seam.
Note: The master cylinder reservoir on Dave's '73 SB has two chambers and two fluid lines to the master cylinder. Rob's '68 Bug has only one chamber in the reservoir and one fluid line down to the dual master cylinder.
Rob -
Sure sounds like it's full enough. And if yours DOES have two separate chambers in the reservoir, you'd see the drop in one compared to the other for sure (unless the chambers are only half chambers, with both sharing the "top" of the fluid).
The master cylinder on my '70 Bug is the original, and it seems to be in good shape - 248,000 miles so it's had lots of work.
I must say I've always been impressed with the brakes on this car. The disc calipers and cylinders have never been off the car either - it still has the original cylinders and rubbers in there and has never showed a trace of leaking. The rear slave cylinders have been replaced once. The disc pads have lasted over 50,000 miles and the rear shoes about 80,000 miles. The weather here is kinder - less slush and no road salt anywhere, so I guess the master cylinders last longer.
Dave wrote –
I don't think the master cylinder has worked properly on this car since we bought it. Might it have gone bad without leaking brake fluid? What if the seal on the first plunger was shot such that the brake fluid was passing around the plunger as it was depressed?
Rob responded –
If the seal on the first plunger were shot it would have the effect you describe. It would also result in a lot of fluid under the rubber cap around the pushrod (behind the brake pedal). You could look for a lot of wetness around there and on the floor if this was happening -- under that cap is the only place that fluid could go, then when you lifted your foot off the pedal some would leak around the pushrod rather than squeezing back into the cylinder.
Someone wrote –
After we got the engine oil leak repaired (oil cooler seals), we noticed a clear oil leak. I assumed it was engine oil since we had just had it changed and the oil was clear. We finally found out that it was the master cylinder leaking. The fluid actually ran down a channel in the center of the car and leaked out the hole in the center of the car, roughly under the rear seat.
At first I was puzzled because the master cylinder itself was absolutely dry. None of the fittings leaked. Once I pulled it off, I could see the fluid between those two walls. The master cylinder was leaking in the little space between the firewall and the inner wall where the master cylinder and brake pedal come in contact. There is a cavity between the two walls about the width of the rubber boot on the master cylinder. When it did leak, it would run down a channel in the center of the car and leak out that hole. If you look under your car, there is a section running down the middle that is about six inches wide. The master cylinder reservoir would run completely dry, and the brakes would become mushy, but still stopped the car. We did have to bleed the brakes after replacing the master cylinder.
Dave wrote -
I didn't realize how bad the master cylinder on our '73 Bug was until I had it off the car. The piston inside was badly corroded, and one of the metal tubes that the bolts run through between the body pieces was missing and the other one was badly bent. I made new stainless steel ones out of a pushrod guide tube from a standard car.
Dave asked regarding installation of the new master cylinder -
Do you know how to turn the fluid line nozzles on the top of the master cylinder around? There isn't be sufficient clearance with the rear one facing the way it is.
Rob responded -
I'm not sure about this one either. If I remember, on my (original) master cylinder, these are screwed in with a locking nut, so would be easy to move. Yours look like they are in rubber grommets, so maybe they will just twist with a bit of force. I take it these are the inlets for the reservoir (and the outlets are the ones "head on" just above the switches.) If so, they don't have to be screw-in, as they only have a "head" pressure of about 18 inches of pipe from the reservoir to contend with.
Note: Dave found that the nozzles turn quite easily and can be properly positioned to provide the needed clearance.
Dave reported -
My friend and I installed the master cylinder tonight without a hitch, then bled it just like we discussed.
Note: Dave found that bleeding the master cylinder with it installed in the car just didn't work. A brakes professional helped him bench bleed the master cylinder, and as a result of this experience Dave wrote a Bench Bleeding procedure that has since been uploaded to our main Web page.
New Master Cylinder Installed
Rob -
Obviously your home-built spacer did the trick. I was thinking after I read that one the spacers was missing, that perhaps this was partly why your pedal travel was excessive. Maybe the floor distorts a little without the spacers, and so the pedal travel/gets spongy.
Dave -
Once I get the push rod adjusted, how do I get the master cylinder boot pulled back over the nut? The boot is inverted a bit now.
Rob -
I think it's supposed to stay "below" the adjuster (don't remember ever having to move the boot to adjust the rod on mine). A bit inverted probably won't make any difference to the boot. It's only to keep dirt out of the cylinder (and fluid out of the car).
After replacement of the master cylinder, Dave was still experiencing excessive brake pedal travel.
Dave wrote -
I'm going to pull both rear brake drums and look the situation over carefully.
Also, I noticed that when I depressed the brake pedal the brake warning light on the dash came on. This indicates that one of the two brake circuits may have failed. The owner's manual says, "The other brake circuit will still operate, but a longer distance and greater pedal pressure are required to bring the car to a halt." Describes our situation exactly.
I screwed the pushrod in until it was just touching the piston in the master cylinder, then backed it off a bit, per the John Muir instructions. (See our Pushrod Adjustment procedure.) Is it possible to ruin the master cylinder by having the pushrod improperly adjusted?
Rob responded -
I doubt it --you'd simply push it into the bore slightly and reduce the normal pedal travel somewhat. Your method should be OK. I simply check to see if the pedal has a tiny bit of real free play before it starts to push the pushrod -- use your hand at the top of the pedal -- it's easy to feel.
Dave -
My brake specialist is pretty well convinced that I didn’t do a thorough enough job of bleeding the master cylinder, and when the air in their gets hot it pressurizes the system.
Rob -
That is a possibility, and I guess the natural sponginess of drum brakes (as compared to discs) means you may not notice a tiny bubble of air by "feel".
The master cylinder is known to be difficult to bleed, so that would makes sense from that perspective too.
Dave wrote regarding pressurization of his brake system -
We've had this problem with two brand-new master cylinders, so I don't think it's a failed circuit (sure hope not!).
Rob responded -
I wasn't thinking that it was a failed circuit (but now you have me wondering!!!) - I was meaning that with two good circuits and the pedal working near the floor, a failed circuit would result in the pedal hitting the floor before the good circuit worked. When one circuit fails the pedal has to push the "dead" piston down until it physically touches the "good" piston (or push BOTH pistons until the bad one bottoms out -- depends on which circuit fails but the effect is the same), so the pedal moves further down before you get any brakes.
Dave wrote regarding the possibility of a failed circuit -
Shouldn't a failed circuit have been evident when we bench bled the master cylinder? Or if there's a break in a line or a bad wheel cylinder, wouldn't we be getting brake fluid leaking out and the level dropping in the reservoir?
Rob responded -
In most cases yes. But I guess it would be possible for a bad piston seal in the MC to leak fluid back up the line to the reservoir and you wouldn't see that. Unlikely since you've changed it twice -- just thinking out loud. I was thinking about this driving home last night too -- you can check the rear brake circuit as follows -- with the car stopped, pump the pedal a few times and get a feel for how far the pedal moves. Then pull the hand brake up hard and then try pumping the brake pedal again. If the rear brakes are working properly, the hand brake will have removed any slack in the system and the brake pedal should work sooner (less freeplay) after a pump or two. If the pedal feel does NOT change, then there may be a problem with the rear hydraulic circuit (rear piston in the MC, lines or slave cylinders).
Can't do this test with the front brakes of course -- no hand brake to take up any slack.
When you've diagnosed that you need a new master cylinder on your VW it's time to get to work. Jack up the front of your bug and put it on jack stands. On a Super, I use the 2 big holes underneath the trunk area up front and use my wide footed jack stands in these holes. Seems to work rather well.
In attempting to locate the master cylinder, you'll need to visualize where your brake pedal is and then crawl underneath the bug and look in this area. You should see a gizmo with metal lines leading to it as well as wiring and rubber lines. That is what you are looking for. Pull the 2 sets of 3 wires off of the master cylinder while you are down there. They should pop right off if they aren't corroded too bad.
Next, I siphoned out all the brake fluid from the reservoir so that when I disconnected the fill lines from the MC, I didn't get a flood of fluid all over my face and arm. Back down under the bug you go, armed with a metric flare wrench (I got away with a close match in a standard size. I think I used a 3/8 wrench but once again, I don't look at sizes, just whatever wrench fits. You will probably need a crescent wrench and possibly a flat bladed screwdriver and knife. Using the crescent wrench, you can remove the brake light switches. They have a big hex fitting on them so that you can remove them fairly easy with a crescent wrench. Then using your flare wrench, remove the 2 brake lines entering in the driver's side of the MC and above where the switches were at. After that is done, remove the 2 lines leading into the top of the MC that run down from your fluid reservoir. If these are stuck, you can use a flat bladed screw driver to work them off, a knife to cut off the lines (and replace with new ones later!) or you can just pop the whole nipple off the top of the MC and removed the nipple later as your replacement MC will already have the fill nipples on it.
Next is a tough part. You do well do get yourself a 13mm impact wobbly socket. To make things easier, you may want to pull your front seat out because you need to get to 2 bolts directly in behind the brake pedal. I used my impact wrench with the wobbly socket and the bolts zipped right out but the MC stayed in. I've done this with U-joints, sockets and wrenches before but it was a PITA! If you noticed your MC is loose while you are removing these bolts, put some rags underneath it so it won't fall of onto the pavement. This baby was worth an $8 core to me and nothing if it got broken. I got down underneath and wriggled the MC loose.
That's all there is to removing it.
Putting in the new MC is a little more involved. You will notice that down around where the MC fits in, there is just 2 pieces of sheet metal between you and the outside world. If you reinstall the MC with no supports between the 2 pieces of sheet metal, you will actually squeeze the body together - tearing or stretching sheet metal in the process. At some point in time, someone did this to my bug and so the metal is scrunched in a little bit. I took a measurement of the space between each piece of sheet metal and cut me some aluminum tubing that was larger than the bolt hole in diameter and just long enough to fit between each piece of sheet metal so it wouldn't scrunch in any more. It was very, very tough getting the little aluminum tubes in between the sheet metal while pushing the bolts through without dropping the little tubes down inside of the body. (BTW: I used a magnetic pickup tool to attempt to look for the original ones and found nothing.) I found that a little piece of wire wrapped tightly around the tubing and bent at an angle, let me hold the tubing inside the sheet metal gap while pushing the bolt through. Once in, I held the bolt in tightly while pulling on the wire and it spooled right off of the tubing.
Next you should really bleed the MC before installing it. My MC came with little plugs (with hookups for the tubing)and tubes so bleed it. I took the fluid reservoir out of the trunk and plugged it directly into the MC and then ran my tubes from the plugs into the reservoir. This forms a little recirculatory system to fill the MC with fluid and not let any air in. Putting the MC in a vise, I was able to depress the plunger in the MC with a bolt. After a few pumps, it was bled. I put in the orifice plugs (little green caps) back in the holes and later removed them one at a time while I hooked up the brake lines and switches.
I put my reservoir back in and hooked up the lines to it. I then had my grandfather hold the MC up underneath the dub while I attempted to thread in the bolts & center the actuating plunger rod into the MC. This was also tough because you are trying to thread bolts, center the plunger, and keep the bolts from backing out and dropping their little sleeves down inside the body. Once we got these started, I loosened up the actuating rod on the brake pedal and backed it off so I wouldn't damage the internals of the MC in case the rod was maxed out on the adjustment. There are 2 nuts. The one closest to the brake pedal merely locks the rod down so it can't move out of adjustment. The one closest to the MC, is the actual adjuster. Once you have the MC snugged up and installed, you run the brake pedal with your hand and you should feel some freeplay and then a part where you meet resistance because the plunger rod is ready to actuate the MC. This gap between freeplay and the moving of the MC should about 3/16 of an inch if I recall correctly from the manual. Use the forward nut to make the adjustment (in or out) as needed, the rear most nut to tighten it down so it won't move.
After the MC is in, get back under the dub, hook up the fluid reservoir lines, your brake lines, and the brake light switches. If your reservoir lines are moist and damp, you can replace them with suitable SAE rubber brake line hose available by the foot from your flaps. I used hose clamps to make sure I wouldn't be losing any fluid as well.
Now you must bleed the brakes. I had help to do this - much easier this way. Get all 4 wheels off the ground and remove all the tires. Since my Super has drum brakes all the way around, the actual bleeder screw sticks out on the top backside of the drum at the top right next to the brake line. I believe it's an 8mm nut.
I filled the reservoir and then went around adjusted all the brakes. (this is a whole 'nuther procedure) Starting with the RR drum, I bled the brakes in the order that they are farthest away from the MC. You do it in the order of RR, LR, FR, FL so that you get all the air our of the lines and one line won't corrupt the other line with air after you have bled it.
Using a tin can and rubber hose capable of slipping over the bleeder screw tightly, I puy my wrench on the bleeder screw, then the hose, and instructed my helper to pump the brakes until he got a little pressure. When he did get pressure, he kept pressure on the pedal, I loosened the bleeder screw, air and old fluid came out of my rubber hose and into the tin can. Then my helper let me know when the pedal touched the floor. I tightened the bleeder screw and repeated the whole process until I had new fluid coming out from each line and no air. It took a total of about 3 times around the whole car before we got it all bled properly because we had induced quite a bit of air in the system. Once you start generating fluid in your tin can, keep the end of the line in the fluid - you can see air bubbles this way and it prevents air from sucking back up into the brake system.
After putting all the tires on and lowering the Super off of the jack stands, we took it for a ride and had much better brakes.
Dave wrote -
I noticed that the rubber portions of the brake lines are a bit the worse for wear, so I guess I'll be replacing them sometime soon. I can't bear to leave the old rubber brake lines -- they are starting to crack, and besides, they SHOW if you look behind the wheels! :-)
Rob responded -
They should be OK for the moment though. If they swell internally (does happen with them as they get past their use-by date) you'd be getting sticking brakes (not retracting) or uneven braking, or if they leak externally you'd see a mess of brake fluid around the area, and the leaking line would be dripping wet.
Dave -
I've decided that I'm going to replace the rubber brake lines with braided SS ones. The blurb in the RMMW catalog says, “... remarkably firmer pedal pressure under all braking conditions."
Rob -
Maybe they are trying to say the older style lines allow for some flex and a softer brake pedal. Could be true I guess.
Dave wrote regarding installation of the braided SS brake lines -
I found that it's easiest to leave the connections loose until the two ends of the assembly are tightened, otherwise it's virtually impossible to bend the lines around the way they're supposed to go.
Dave wrote –
My brake pedal goes a long way down before there is any braking action. There is no sponginess.
Rob responded –
Check the fluid reservoir quick -- you might be on only one set of brakes. It sounds like one half of the dual brakes is not working, as the symptom you described is exactly right for half brakes.
Normally the rear brakes come on first (the piston inside the master cylinder closest to the pedal) and when he rears come on the continuing pressure from your foot pushes the rear cylinder a little further and applies fluid pressure to move the front piston down. If the leak is in the rear, the rear piston moves forward until a "nose" on it contacts the front piston and applies the front brakes. If the leak is in the front, the rear piston keeps moving down until the front piston bottoms out and the rear half then gets pressure from your foot.
Either way, you get firm brakes with your foot near the floor, but on only rear or front drums.
Dave –
Is it possible that only one of the two circuits in the master cylinder is working, for some other reason besides a leak?
Rob –
Yes -- but that would simply prevent the fluid in that circuit from moving and so the pedal would go less distance, not more.
I guess it's possible that the few times you needed the brakes after it happened has not leaked much out of the reservoir -- just lowered the level a little.
You can either go the whole hog, or just do it section by section. For info, the rear cylinders on my car are Brazillian VW, and work as well as German stuff. Been in the car for maybe 8 10 years, so they are lasting just fine too. Some parts I won't touch from South America, but other parts seem just fine.
Dave wrote -
I had a look at the master cylinder. It does indeed have two lines coming into it from the fluid reservoir. It looks really cruddy and the wires are all exposed -- I am going to go ahead and replace it, I think.
Rob responded -
It MIGHT be fine internally, but looks like it's getting past it's use-by date doesn't it. Good safety plan to replace it, I think. I'm sure my brake light switches have only two wires on each -- you have four -- very interesting. Must mean there are at least two types of switch.
Dave asked, regarding the switches on the master cylinder -
Are both switches activated when the brakes are applied?
Rob -
Yes -- either switch will operate the single brake-light circuit -- they are mounted in parallel. Two switches mean that even if you loose fluid from one brake circuit (front or rear) entirely, you still get brake lights.
Dave -
I checked the master cylinder first and found what I hope was at least contributing to the problem. The brake light switches were leaking.
Rob -
Sounds like that was the major part of it. When you held your foot on the brake did it slowly drift down towards the floor? If so that would indicate a leak under pressure, which loose brake switches would fit nicely.
Question -
I have a 1986 Mexican beetle … the stop lights, don't work, I checked the fuses, and the connections, and everything seems fine. I think is the switch in the master cylinder, but I don't known how it works.
Response -
The master cylinder has two switches in it (I'm assuming the "modern" Mexican bugs have similar master cylinders to the older German beetles). One switch for rear brakes and one switch for the front brakes, so if one brake line fails you still have half your brakes and your stop light still work.
The switches are in parallel - either or both will work the stop lights They screw into the master cylinder, and are easy enough to replace. You just pull back the rubber protector boot, pull off the wires, unscrew the switches and replace them. You will probably have to bleed the brake lines afterwards - some air will probably get into the lines. A spongey feel to the brake pedal is a sign of air in the brake lines.
Someone wrote asking about the interchangeability of master cylinders.
Response -
If you are in the US, then the master cylinders for '69 and '73 should be identical (they are certainly interchangeable anyway).
If you are in other parts of the world (like here in Australia) which got disc brakes up front from '67 onwards, then the ORIGINAL master cylinders for these are a little different (different compensator/relief valve or some such) to the drum-only models.
BUT - the replacement master cylinders available these days re "universal" - they will work fine for both drum and disc brake models. So even if the replacement looks a little different in detail - it should work just fine.
My '68 Bug has only one brake light circuit for the master cylinder, but it was either '69 or '70 they introduced the twin switches, so it's just possible your's has only the one switch (if original). If this is so, then a simple "parallel" wiring from one switch to the other fixes that problem.
Question -
I found that the rubber gaskets/fittings on the top on the cylinder that take >brake fluid from the reservoir lines are moist/weepy. Not necessarily gushing fluid by any means, but they are wet. Bench bled the unit as per instructions, then did the whole system to bring up pressure. System is working fine, but I'm wondering about that wetness at the fittings.
Response -
I'm not quite sure how to answer this one -- the master cylinder should remain dry on the outside. I guess just keep and eye on it and if it continues to remain fresh-wet, and maybe the fluid level starts to drop in the reservoir than you have a problem. It may be just trace wetness that won't continue. If I remember right, the plastic fittings on the master cylinder are a push-fit, and can be rotated a little to line up the feed lines easily, so they should be just push-tight.
The important thing is that the brakes are working well and you can use the car.
Question -
I am having a problem with the master brake cylinder in my ’69 Bug. I went to "Auto Zone", "Pep Boys", "Advanced auto" but everyone has the same cylinder which doesn't match mine. I have a three-switch master cylinder and everyone has a two switch replacement????... I have three two-pole switches. I thought I read somewhere about VW switching to two three pole switches. Got any advice?
Response -
Neither Dave nor I have experience with three-switch master cylilnders. I would recommend that you visit Speedy Jim's Home Page, which has information on the three-switch master cylinders.
Question -
I bought a rebuilt master cylinder and it immediately started leaking so I ripped it out and took it back and am now waiting for a good one from California.
Response -
Read Dave's article Bench Bleeding the Master Cylinder on our Web site before you put the "good" master cylinder in the car - it's almost impossible to do it properly in the car with a new master cylinder, and you HAVE to get every trace of air out of the master cylinder and brake lines for good brakes.
Disclaimer stuff: Rob and Dave have prepared this information from their own experiences. We have not assumed any specialised mechanical knowledge, but we DO assume that anyone using this information has at least some basic mechanical ability.
We hope you find this information useful, but we don't take any responsibility for anything which happens to you, other people, your VW or any other property or goods resulting from your use of this material.
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Last revised 26 August 2004.
