There doesn't seem to be much info on what to do with a leaking PDWA anywhere on the 'net. One exception is at http://www.vtr.org/maintain/pdwa.html . My experiences were slightly different.
Warning: I am not advocating changes to your braking system. If you choose to follow a similar procedure, YOU are solely responsible for understanding and living with the consequences of your actions.
First of all, what is a PDWA? It stands for Pressure Differential Warning Actuator, a mouthful just crying out for a catchy acronym. Basically what this device does is warns of a hydraulic failure in one (front or back) brake system by sensing the pressure differential (the good line holds hydraulic pressure; the bad one bleeds off pressure through the leak).
Here's how it works: the PDWA is mounted on your firewall just to the side of your brake master cylinder. It has the twin brake pipes from the dual m/c running into and out of it. Basically it is comprised of three chambers; the left and the right chambers which are full of brake fluid and the center chamber which should be dry. Although the chambers are interconnected, there is a piston (or two, depending on the model) with "rubber" o-rings that seals off the center chamber from the outside two. The piston has a groove running around it and the plunger of an electrical switch sits in this groove. As long as there is no leak in the system, the pressure on both sides is equal and the piston remains undisturbed. However, if one braking system develops a leak (say a wheel cylinder), the pressure on that side will be unable to build up as high as the good side. This pressure differential will push the piston to one side, the electrical plunger will be lifted out of the groove and a warning light will go off on the dash.
While the theory sounds pretty good, especially if you say it real fast, reality is a little different. The one time that I had a brake failure from a badly leaking wheel cylinder, the brake pedal went to the floor and the car didn't stop (just like opening a bleed valve). I didn't need, nor did I even notice, a l'il red light glowing weakly on the dash to let me know that I was in a heap o' trouble. I've also had slow leaks from wheel cylinders that were given away by a brown puddle on the garage floor without any special guest star appearances from Mr. PDWA Red Dashboard Light Man at all. So, based on this admittedly limited empirical sampling, you need a kinda bad, but not too bad, leak for this thing to be useful. In other words, I believe that the chances of this thing actually being useful are remote. (YOU need to decide if YOU agree.)
Being useless would be OK, if ol' PDWA just sat on the firewall, minding his own business and never giving no one no trouble. But, at some point you may notice the PDWA sitting in a puddle of it's own making (bladder control problems?) and then you're faced with some choices. The leak is almost certainly coming from the electrical switch which is not designed to be fluid tight. This means that fluid is coming into the central chamber - the two most likely causes being 1) failed "rubber" seals or 2) pitted cylinder. You now have three choices:
Buy a new PDWA (expensive and hard to find)
Rebuild your PDWA (more on this)
Bypass your PDWA
The PDWA is relatively easy to dismantle and rebuilding would seem to be a viable option. Unfortunately if the cylinder is pitted, you will need to get it re-sleeved, which is pricey. If it is only the o-rings, that would in theory be an easy fix. However, you MUST find o-rings that are made of a material that will not break down in brake fluid and that is not easy to do. Most o-rings that you find a plumbing shop are NOT suitable for submersion in brake fluid. They will fail in short order and you will be back to step 1.
I elected to bypass my PDWA, by leaving it in the loop, but by basically defeating it. There are at least a couple of ways to do this. One fellow spitfire owner had tapped the walls of the center section and replaced the moveable piston with an immoveable threaded rod. I liked this idea, but I didn't have a tap large enough. Another fellow owner had simply epoxied over the electrical switch, sealing the leak. I went this route and have had good luck. The downside is that to a certain degree, you may defeat the dual braking circuit. This is because you have not addressed the leaking o-rings or pitted piston and fluid is able to pass between the two circuits. I decided that this was an acceptable situation since a) I haven't found that the dual circuit was all that effective in case of brake failure anyway and b) I believe that there will be sufficient pressure held by even deteriorated o-rings for the one necessary emergency stop and c) good maintenance and careful attention to the brakes makes a brake failure remote anyway.
Hopefully this will give you some alternatives to think about for making an informed decision should your PDWA begin leaking.
Disclaimer: This is not an endorsement for Napa Auto Parts, but a description of a part that worked for me. I hope that this will be helpful for fellow enthusiasts, but I make no representations as to the suitability of this part as a replacement for stock.
For some reason, my rear lug nuts seem to fail regularly, either snapping off, cross threading or just stripping and refusing to tighten. Maybe I'm ham-handed, but 3/8 inch just doesn't seem hardy enough. In addition, the Triumph replacement parts are pricey and you can easily spend over $100 to replace the entire car.
Realizing the advantages - price, availability and strength, I decided to upgrade. I found a great article on upgrading here:
http://www.oocities.org/MotorCity/Speedway/1080/lugupgrade.html
I was all ready to go with this, but I was unable to find the lugs that Peter used at any of my local discount auto parts stores. I noticed that a comment had been added at the end of the article that Ford Taurus lugs will work, but no details on which ones (Taurus lugs vary by year and even by front to back). So I went to NAPA with the sizes and the guy behind the counter was nice enough to go through a number of cross reference books before coming up with this Napa stock number: Dorman Wheel Stud 610-344. Prices varied (I had to go to a couple of different stores) from $1.40-$1.60 each. This is actually a metric size (12mm X 1.5) and differs from the specs in Peter's article as follows:
|
Original Article |
Ford Lugs Metric |
Ford Lugs English Equivalent |
|
|
Knurl Diameter |
0.525 |
13.59 |
.535 |
|
Shoulder Length |
.563 |
11 |
.433 |
|
Overall Length |
1 3/8 |
46 |
1.811 |
Apparently the 1/100 inch difference in knurl diameter (the portion that is "ribbed" and fits in the hub) is insignificant as these fit tightly, but not overly so.
Installing them was a bit of a project as they are too long to fit in from the back even after removing the brakes. And don't bother removing the wheel cylinder or adjuster to gain additional room - I tried and it doesn't help. (Note that as of this writing, I have only upgraded the rear wheels. I presume that the front wheels will be much easier since the hub can be removed from the rotor. If anyone does this, please let me know the results.) You may need them to be as long as possible - more on this later - so carefully grind or cut them down the minimum amount possible. I would actually wedge them partially in against the backing plate and them tap them into place, probably putting some momentary stress on the backing plate. While I was cutting the lugs down, I left a bolt on further down. I could thencarefully remove this bolt to clean up the end threads where I had cut - if you have a tap, even better. Once the lug is in the hole in the hub, it can be pulled forward by stacking washers (or I used an oversized bolt) and then tightening down to force the lug forward into place.
I would recommend replacing one lug at a time as a long pry bar through two of the other lugs wedged against the ground will prevent the hub from turning as you tighten the bolt to pull the lug into place.
As far as the wheel and brake drum go, the holes are large enough to accommodate these lugs and no further drilling is necessary. I did note the heads of the lugs are larger than stock and come uncomfortably close to the slave cylinder, a little gentle attention with an angle grinder took care of this (interestingly, only one wheel had clearance problems - probably depends exactly where the slave cylinder is floating - but I went ahead and ground both sides down.)
I have a stock 1500 wheels and I wanted to keep the wheel trim plates. There was no way that I could figure out to do this with upgraded lug nuts, the stock wheel nuts have a special shoulder to hold the trim plates in place that doesn't seem to be available in the upgrade size. So what I did was hold the wheel in place with normal lug nuts, put the trim plate OVER them and then hold the trim plate on with chromed lugs. This is why you need the lugs to be as long as possible; I cut the first set down to same length as stock and there wasn't sufficient threadage left to hold the trim plate on.
Of course, this does not apply if you have an earlier model round tail or non-stock wheels or have discontinued using the trim plates anyway.
A package of lug nuts is about $2 and the chrome lugs nuts about $5 for 4. Make sure you get the tapered ones that will locate both the wheel and the trim plate. Here is a picture that shows the final result (as well as my rust and sagging spring.)
Some further info from Brad Richardson (thanks Brad!)
We used 7/16 x 20 wheel studs from NAPA, Balkamp BK 641-1063(R). The only 'modification' we had to do was for the ones on the front of the car. Had to carefully, (did it by hand) grind just a tad off the head, because the back of the rotors is curved up a little. Other than that, they just popped in, and we used standard 7/16 lug nuts off the shelf in a local auto parts store. Length was perfect, and knurl size, etc. Of course, we were putting these on for non-stock mag wheels, which had PLENTY of room for length.
We found this size by going through NAPA's "paper" (not on-line) catalog and carefully measuring all the actual size pics until we found these that were as close to the exact size that we could find. My calipers couldn't tell the difference once we actually had the parts in hand, for the knurl. Obviously they are 7/16 instead of 3/8, so a difference there, but no drilling on the rotors or rear either.
More info and parts numbers from Walt on the Spitfire mailing list
On another web site (don't remember which) you will find referenced a Wheel Tite bolt P/N 28020.
I located these at the local Pep Boys, a few on the shelf, but plenty in the warehouse, (according to them)
Wheel Tite
P/N 28020
7/16" X 20
Knurl Dia. .525
Shoulder. Len 9/16"
O.A.L. 1 3/8"
Packaged at Motormite Mfg. Div of R&B, Inc
P.O. Box 1800
Colmar, PA 18915-1800
(in 1992)
Some grinding of the head for wheel cylinder clearance may be required (rear).
I chose to go with 12mmX1.5, 13.15 knurl, L-43 shoulder 11 (all mm) found in NAPA's picture catalog P/N 641-3128, with chrome acorn nuts P/N 735-2168 for my particular application. No grinding necessary, tapered head, pressed right in,for use with TR7 wheels.
From: Barry SchwartzFor those of you interested, Auto Zone is another source for 7/16-20 wheel studs. In fact for me, it was the only place that had them. Pep boys (three different ones) couldn't find them under any brand name or car model wheel tite number 28020 or Dormans number 610-175) Neither could Kragen. None of the tire places would even sell just wheel studs For Auto Zone I just told them I needed a FRONT wheel stud for a 1972 (71-74) Mercury Capri. The part number he came up with was 98002. They fit perfect and they don't need any modifications either!
