Texas RS

I have a brake fluid leak that appears after I have been on the track. The fluid is coming from the brake fluid reservoir vent. That is at the top of reservoir. The drain line attached that runs down to in front of the left front tire. There is one additional symptom that May or may not be related and that is on cold start up after the fluid leak the pedal will go to the floor, no brakes at all. With just a single pump the brake pedal is hard as it should be.

I just put a brand new master cylinder on it. Now I am wondering if the brake booster could be causing the problem.

Amber Gramps

Sounds like you answered your own question. You may have a leak in the booster.

rusnak

Does the brake fluid level stay the same, or does it drop like there is a leak?

Texas RS

I am not familiar with how the booster works, or in this case fails. What causes it to force fluid out of the reservoir?

Yes, the fluid level drops. The reservoir was nearly empty after two sessions on track.

Droops83

So, you had this symptom before replacing the master cylinder, and it is exactly the same after replacement?

My first guess would be a master cylinder issue, but you replaced it . . . . what brand? What method do you use to bleed brakes?

A leaking brake booster generally does not cause the brake pedal to go to the floor; in fact it usually causes the opposite, with an overly stiff brake pedal. You can check it by pressing on the brake pedal (after it has pumped up!) with the engine off and holding it while starting the engine. The pedal should drop slightly once the engine builds up sufficient manifold vacuum (a second or two after startup at most). Listen carefully for an idle change while pumping the brake pedal. Any change there usually indicates a leak in the booster itself or in the vacuum hose between the booster and the engine intake manifold.

The only way a brake booster issue would cause a soft brake pedal is if it fills up with water and the internal return spring rusts to the point that it loses spring rate and/or falls out, and you get an over boosting scenario. I have never seen or heard of this on an early 911, but it happens on E39 5 series BMWs with clogged water drains and on very early Boxsters and 996s (Porsche issued a TSB for a redesigned brake pedal pushrod/seals). Remove the master cylinder and peek inside with a flashlight to check if the booster is filled with water and/or brake fluid.

Your other symptom is a strange one and may or may not be related to the no pedal when cold. The usual cause of fluid leaking from the vent is too high of a fluid level for track use. The brake fluid gets very hot with repeated heavy braking and it expands. This causes the level in the reservoir to rise and sometimes leak from the vent, which is designed for this very scenario. For track use, do not fill the brake fluid reservoir all the way to the MAX mark; halfway between MIN and MAX is sufficient.

Are you sure that the leakage from the vent accounts for all of the brake fluid leakage? How is the braking performance on track? Is the pedal feel/travel consistent for the entire session? No brake fade or boiling fluid?

The pedal going to the floor during the first cold application is strange, and usually points to the master cylinder, but you say this happens only once and the second stroke pumps up the pedal every time. Does the pedal go all the way to the floor, or only halfway?

How old are your brake hoses? A hose that is partially collapsed internally could cause the corresponding caliper to drag and superheat the fluid and thus expansion. Check all of your pads for chunking and check your rotors for excessive heat spots.

Does the car roll freely when the conditions are present for the brake pedal to go to the floor? Check that it does. It is a long shot, but it could be that one or more of the brake caliper pistons are stuck in their bores from heat expansion and the brakes are stuck on. The first cold application frees them up, the brake fluid returns to the reservoir and the pedal pumps up. Such heat could also cause the fluid to get really hot and expand to the point of overflowing from the reservoir. Which is why I asked about on-track braking performance. It can be difficult to get adequate cooling air to early 911 brakes, but there are ways.

The other long shot is there may be debris floating around in the brake fluid reservoir and intermittently plugging the master cylinder inlet bores. If there is enough blockage, it could push fluid back into the reservoir instead of into the brake circuits . . . . . again this is a very remote possibility and your braking performance would not be good at all.

Check all of this and report back. Good luck.

Texas RS

Chris thanks for the comprehensive response.

The pedal goes all the way to the floor, essentially no resistance. It only happens after I have found the puddle on the ground. All of the leakage is coming from the vent.

The brakes work beautifully on the track, good solid pedal no noticeable fade.

All wheels spin freely when I jack the car up.

I have a pressure bleeder, but normally bleed them by pumping the pedal.

In regards to brake overheating, it is possible but the brakes are well ducted as this car was a successful E stock race car before I bought it to use in DEs. All of the ducting is in place and in good shape.

Tim

Amber Gramps

Maybe the reservoir cap isn't venting properly. Fluid in the overflow line may be syphoning fluid from in front of the brake piston.

Texas RS

For this model the vent is not in the cap it is a small connection at the top outboard side of the reservoir, just below where the cap fits. I could see where there could be a chance for fluid to get into the line under hard right hand cornering. I have a difficult time seeing how the fluid level could be as low it is at the end of a run.

Amber Gramps

OK, so the reservoir is stock. Hmmm....

You only have two options then. 1) The fluid is boiling and expanding to the point that it escapes into the cap while you are on the track eventually finding it's way to the ground. Fluid then cools and contracts back down once you are back in the paddock leaving air inside the compression chamber in the master cylinder. You don't notice the fluid is boiling because the reservoir is constantly compensating for it. That is some hot brakes.

2) Something is forcing air in behind the brake piston diaphragm and into the compression chamber. Unlikely but was my first thought.

I did have a shop teacher that had a small chunk of invisible crap that would occasionally block the port between the reservoir and the cylinder causing intermittent dead paddle.

Texas RS

For this model the vent is not in the cap it is a small connection at the top outboard side of the reservoir, just below where the cap fits. I could see where there could be a chance for fluid to get into the line under hard right hand cornering. I have a difficult time seeing how the fluid level could be as low it is at the end of a run.

Amber Gramps

It could be a bad piston seal letting fluid leak out and air leak in. I have no track experience, only 30 years of doing brakes for myself, family, and friends. Master cylinders are an of duck. Is the area under the MC dry?

Texas RS

Oops, sorry about the re-reply, I am on my iPhone and it looked like it didn't post.

Expanding/Boiling fluid - I am using Prospeed 686 brake fluid and I do not have even one ounce of noticeable fade on the track. In terms of motive force it seems something like this has to be happening to get the impact I am seeing. The other observation is the fluid level is at the very minimum (just above the two outlet connections) when I check it. It would require fluid expansion about equal to the reservoir capacity for this to occur.

rusnak

My first thought was that the brake booster was sucking all the fluid out of the reservoir and some got blown out of the vent hose. But I'm not clear on what's different about your vent system when you say "on this model". Is it not stock?

Texas RS

It is stock. The reservoir has a vent at the top near the cap on the outboard side, there is a drain connected to it that drains down in front of the left front tire. The car is in my storage unit right now, so I cannot take a picture until later in the week.

Ed Hughes

If you bled the brakes using pedal after installing the new MC, you may very well have ruined it. Do you pump all the way to the floor when you are bleeding?