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Brake Bleeding

Recent postings re pressure bleeding brakes prompts this response. I too have found that pressure bleeding provides the best "pedal." I have had very little luck with the vacuum type (Mity Vac) and while pedal pumping certainly is thorough, an available SO or co-mechanic is not always an option (not to mention the trouble pumping your brakes during bleeding causes with your master cylinder!). The best pressure bleeders could end up costing several hundreds of dollars and likely will not deliver that much harder of a pedal.

My solution to the brake bleeding dilemma has been to fabricate a rather simple pressure bleeder. While I have been tempted to market this system, I am happy to outline the parts and the concept for my fellow Porsche wrenches (picture).

There are a couple of options for pressure source. As mentioned in other posts, a compressor is a good source of air, but not absolutely necessary. Some commercial systems (possibly the EZ system) use your spare tire as a compressed air reservoir and this could work for you as well. Any suitable storage tank can be used as well.

The most crucial difference in my system, compared to any other that I have seen, is the addition of an air filter. As we all know, brake fluid is hydroscopic (it absorbs water- definitely a no-no when it comes to brake fluid) and thus must be kept dry. Any compressed air will contain substantial amounts of H2O and thus must be filtered before using to pressure bleed. I simply install in line a "last chance" air filter, the type used when spray painting to keep the air dry (costs about $3 at any paint shop).

While the cap for the reservoir can be purchased, I simply used a spare cap, drilled it, and installed a tire valve (picture). Other "options" in my system include an in line pressure gauge (while some people might think pressure bleeding is for "Chevys," if you keep the pressure low - around 10- 15 lbs., you won't have any problems. I also install a regulator valve/shut off.

The key to my system is to seal off the over flow and to NEVER let the reservoir run dry. One way to assure fluid in the reservoir at all times is to "bleed" with a small enough container that you have to empty regularly. Each time you empty the container (which should be smaller in capacity than the reservoir), shut the line off to the pressure, and top the reservoir off.

Enjoy- If any one has more suggestions, or comments (or needs further instructions), please let the e-mail, and brake fluid, flow.

QUESTIONS AND ANSWERS

Q: In a message dated 98-09-22, Thom Fitzpatrick asked me what trouble pumping your brakes during bleeding causes with your master cylinder. Thanks for the follow-up Thom, I suspected someone would ask that question and should have dealt with it then instead of now, but here goes.

A: In "normal" use, the plunger/piston in a master cylinder probably only goes into the cylinder about 1/3 of the way or so. In part this is true because under "normal" use we don't really push the limits of pressure of the system and don't need the extreme pressure the MC can deliver. It is also true because under "normal" use when we use the pedal, the system is pressurized and we really can't push much further without some serious force. When racing, the system looses some of its effective pressure (OK, I am not an mechanical engineer, but boiled fluid, fluid with water or air, or trashy fluid can be compressed more than otherwise, right?). So, in racing, or when bleeding your brakes with the valves open, the piston can freely depress into the cylinder to the physical (non-pressure) limit of the MC.

While new MCs have clean and smooth pistons, pistons on older MCs are prone to get dirty, and yes even rust or corrode. As the piston is depressed into the MC under normal braking, the seals and fluid help to keep that 1/3 or so clean, smooth, and well lubricated. The remaining 2/3s or so is exposed to the air and does not get the benefit of regular cleaning. When one depresses the piston when bleeding and the valves are open, the dirty 2/3s (no cigar comments, please) enters the cylinder and drags across the seals. While it might not be the worst thing in the world in general, the more dirt or corrosion on the cylinder or the more frequent and vigorous the pumping of the dirty piston across the seals, the worst the damage, which leads to a leaky MC.