Switching Your Braking System to DOT 5 Brake Fluid

by several Spitfire Mailing List members

This info was posted by Joe Curry on 1/6/01.

Silicon (DOT 5) Brake Fluid by Joe Curry

When spending countless thousands of dollars, Pounds, Euros, Pesos, Lira or whatever other currency you use on restoring your Spitfire, it is a good idea to consider changing the brake fluid to modern silicon (DOT 5) fluid. In the past, it has always been very important to use only the manufacturer's recommended Girling or Castrol brake fluid to prevent damage to the natural rubber components in Spitfire braking systems. The silicon fluid will not eat the natural rubber, nor will it eat the paint when it is spilled or leaks onto the car's bodywork.

In changing over to DOT 5 fluid, it is important that the entire system is thoroughly cleaned and all remnants of the previous fluid is completely removed. It is also highly recommended that all the rubber components in the system are replaced concurrently with the fluid. If the components in the master and slave cylinders are sufficiently worn, they surely will leak unless they are replaced. However, many times people have gotten away with just purging the system thoroughly with mineral spirits and then pumping a couple of pints of DOT 5 fluid through the system to ensure the old stuff is completely removed.

I followed this procedure on my race care brakes and only one of the calipers leaked after the conversion. It was so successful that I plan to convert the hydraulics on my street Mk1 to DOT 5 the next time I have to do any brake work on it.

If you do this conversion, you will never again have to worry about the paint bubbling off the deck under the master cylinders!

This info was posted prior to Joe's Tip of the Month.

Jeff McNeal asked how to convert from standard brake fluid to DOT 5 synthetic. This fluid is preferable to standard fluid in that it doesn't absorb moisture from the air and also, what is possibly the biggest plus, it doesn't eat paint. Here are several tips from the list.

When I rebuilt my '77, I went all though the brakes and the clutch system. Before installing the rebuilt calipers and the new rear wheel cylinders I flushed the system and then put in silicon fluid. Be sure to flush the system as the silicon fluid is not compatible with the older DOT 3 or 4. I flushed mine with the silicone fluid, others have recommended isopropyl alcohol, which is definitely a better way to go (less costly and less messy).

Paul Mostrom

Jeff, if you are going to use DOT 5 fluid, you will need to flush the entire braking system with Denatured Alcohol. DOT 5 is not compatible with conventional brake fluid, and will cause you much grief if it gets contaminated. You should get at least a couple quarts of Denatured Alcohol, and flush the system before you take it apart. After you take the system apart, blow out all of the lines with compressed air. Also, you should get a set of caliper kits, and rebuild the calipers. You should also replace all of the rubber brake hoses also. Doing these things will allow you get many years out of the braking system. I am doing the exact same thing with the brakes on my 75 Spit, and have done hundreds of DOT 5 conversions. It's definitely the way to go!

John Smith

Replace every single brake seal. There is no way to know what size MC, caliper, or rear wheel cyl you have until you find the part number or measure it. People switch these around all the time. Lest we forget to tell Jeff, perhaps the best advantage of silicone is when a leak at the MC does occur, you won't have to repaint your bulkhead.

Bill Brockschmidt

If you flush with solvent, be sure that you try to use DRY air to blow out the lines. Many air compressors have a drier/water trap, drain it first. The cooling effect of blowing air/solvent will condense any water. Alcohol usually contains some water anyway, and will readily mix with any that condenses. It can puddle in low spots. Dry nitrogen works very well if you can get it, instead of air.

Use plugs to seal the open ends of the brake system (pencils will work in a pinch!), then blow thru from master cyl end, unsealing only one at a time, starting from farthest to M/C. Keep the system sealed at all times except when actually working on that piece.

Overkill? Maybe, but Silicone fluid IS expensive, and you ARE using it to get better brakes. Details and preparation count.

Carter Shore

And if you don't want to switch, Chris Pappathopoulos asked the following:

In one of the posts about DOT5 brake fluid the author said something about just using Castrol Girling fluid and sounded like other DOT 3 fluids would eat up the rubber. Is this true or do you just need any fluid that meets DOT 3 and SAE J.1703d specs as my manual states? Or am I just reading it wrong?

And Joe Curry responded:

You have to be very careful selecting the proper fluid because the British typically use natural rubber seals as opposed to the US's use of neoprene (synthetic). You can feel safe if you stick to Castrol GT LMA fluid. It is readily available.


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