After traveling to the Gathering in my E type last weekend I noted that the brake pedal started sinking toward the floor very slowly when I applied the brakes. I had recently replaced the front rotors several weeks before and had experienced no problems. By the way, this is a 4.2 OTS. The master cylinder was replaced in November
of 2023, also no issues prior to 4/21/24. Also, when the pedal sank, it would only sink so far, then I had good brakes and a solid pedal. I guessed (wrongly) that the new master cylinder was bad and installed a new one. OOPS, same symptoms and now the brakes tend to stay on for a time after I pump the pedal. I have looked for leaks but there are none that I can see and the reservoirs are still full. I have also checked the pedal free play and have about 1/2" before the pedal engages the master cylinder. Rather than throw more money at the problem, I thought I’d put it out on the forum for suggestions. I’m guessing that the booster/servo is bad but, maybe someone has a different opinion.
Pumping to firm up a soft pedal is indicative of air in the system. A mushy pedal can also indicate brake fluid getting past the seals in either the master cylinder or slave, though no apparent fluid loss in the reservoirs might rule that out - you might otherwise find brake fluid from the m/c in the carpet or accumulating in the vacuum booster. We may need additional details. What kind of brake fluid are you using? DOT5 is notorious for air entrainment and if you have remote bleeders in the rear it’s sometimes difficult to force the bubbles downward and out of the system.
There is no mush in the pedal. I also checked the rubber brake lines for inflation but that is not the case either. As the pedal is sinking, it feels like it is forcing fluid through a small orifice.
Booster problem, but maybe caused by vacuum issues. It’s a difficult system to diagnose, and sometimes you end up just changing things. I’d start by just tightening the clamps on the vacuum hoses. It could be that simple.
That’s an interesting comment Mike. I put the vacuum gauge on it yesterday and found 12 inches of mercury (at 500 rpm) instead of the 17 to 20 I was expecting. I chased a vacuum leak several months ago and replaced all the hoses and clamps. I’ll go through it again.
You are not loosing fluid, so not a caliper leaking and the mc and slave are not leaking externally. You have internal seals leaking.
If fluid was bypassing the mc internal seal, the pedal would almost certainly go to the floor. Since the pedal goes part way, it is an indicator one of the two slave piston seals is bypassing. From what I hear you saying, I would rebuild the slave cylinder.
Tom
One thing that can easily cause that feeling is a de-laminated flex hose.
I spent some time yesterday replacing the 3/8" vacuum hose with 11/32". It seemed to make some difference. I also changed the ignition timing using a vacuum gauge and got the needle into the green by advancing the timing. Haven’t driven the car yet to test the results but I did get a nice puddle of brake fluid on the floor that was leaking from the rear of the booster. Guess I know what to replace now.
Leak on the floor is more likely coming from either the brake or clutch master cylinder. Guess it depends which fluid reservoir is down…
I have now replaced the booster/servo and have the same symptoms. Here’s my theory after thinking about this for a week.
The booster/servo seal lost integrity allowing air to enter it originally causing the pedal to sink and pumping brake fluid into the servo to displace the air, but it does not do this, the air remains. Since I am not using the brake pedal to bleed the brakes, the booster is not activated and does not move the air out of the servo and the master continues to try to displace the air with fluid, Has anyone found that the system bleeds better when the engine is running? I noticed a section in the shop manual that says open the port on the slave cylinder, and bleed normally. Has anyone had success with this method?
The booster has no direct connection with the pedal. In rest state, the servo isn’t filled with air, it’s under vacuum. If the seal was failing on the slave, the booster would have been partially filled with brake fluid.
It sounds like you need to bleed the master, which can be done as you say by opening the port on the slave. I find that having the engine running helps when you’re trying to bleed, but it always seems to get an argument.