When the brake is applied, It remains soft until it is pumped once. It gets hard the second time (good braking power at that point). It becomes soft again as the pedal gets released for a few seconds. This may seem like air in the system but all 4 brakes were thoroughly bled multiple times, using a vacuum system and the bleeders were replaced with ones with a check valve. The brake oil in the jars remains at the same level so I do not believe that there is a leak in the system.

Here is the rest of what was done to trace the problem:

The booster was replaced, Master Cylinder was changed, Flexible hoses at all 4 positions replaced. All the wheel cylinders (calipers) were also replaced. In addition, I did a vacuum test (applying suction on 1 hose and plugging the other) on the vacuum tank and it seems to hold vacuum.

I am at a loss. Any ideas or tests would be welcome at this point.

Thanks

Submitted by jos@stoop.org on Mon, 11/26/2012 - 10:55

Francois, I would also love to hear if you found the cause of the problem.

I had the exact same problem with my 1965 S-Type. Last night I spent a long time bleeding the system, but couldn't resolve it. The brakes still push to the floor on the first press, and are pretty hard on the second. I still believe there is air in the system though, when I finally gave up there still remained some fine bubbles.

I have bled many brake systems but never saw the behavior from last night: The brakes were working well, except for the problem stated above. I started bleeding at right rear caliper, having an assistant pump the brakes three times, then opening the bleeder and observing the brake fluid through the clear hose, before closing the bleeder and then letting the assistant bring the pedal back up. Foam was coming out right from the beginning and still after bleeding about 10 times. Went to the other rear caliper, and saw the same thing. Air must be getting in the system from somewhere. Decided to do the front calipers - to see if this was specific to the rears - and it was different but worse: it would have clear fluid followed by very large air pockets, then clear fluid again, each stroke of the pedal. Same on both front calipers. Note that the reservoir was filled with fluid at all times, being regularly refilled, no brake fluid was observed leaking from anywhere in the system other than where bleeding.
I finally decided to bleed with the engine running, going around at all calipers again (starting at the front), which reduced the air coming out to just minor bubbles but never saw just clear fluid.
Went for a drive last night, and found that the brakes behaved exactly as before - need two pedal pushes to engage the brakes- except that now the brake booster seems to not work either or at least not work very well. The brakes seems reasonably hard though and engage well (at lower speed). I am guessing that air is introduced at the either the master or slave piston moving back or forth. Unless it is suctioned into the hose along the bleeder screw thread at every bleeder (even though I only open them less than a quarter turn), it seems air is definitely getting into the system, which is a problem. It puzzles me is how it could be enough air to cause long pedal travel but the brake not to feel very spongy. Perhaps there are two separate problems.
This is a car that was completely restored some 8 years ago, everything looks very clean, new calipers, rubber brake lines etc. I read somewhere that mismatched components (larger rear calipers with original master cylinder) could cause the double pedal push problem. I just bought the car so I don't know but can't imagine it had this problem for all that time since restoration.
Any suggestions appreciated.
Jos

Submitted by fmasbou@netzero.net on Fri, 11/02/2012 - 21:25

I use standard DOT 3 as a fluid. I have also bled the btrakes the "old way" multiple times until I changed to a check valve type bleeder to make it easier. The booster is a new unit. The old one was probably fine but I replaced it with a new one in the hope of correcting the problem.

I like the idea of the travel of the calipers but how could you control the travel of the calipers? Would you not think that you would get uneven braking if one was not filling up in one push (assuming the 3 others would)? I will look at them anyhow.

Submitted by bonnettoboot@e… on Thu, 11/01/2012 - 22:36

This does not seem to be a fluid problem it is a mechanical issue. Somewhere in the system ther is too much of a gap for the master to fill it in one push. You have to check all of the callipers, one likely has too much travel before applying.

Submitted by phil19564@1usa.com on Thu, 11/01/2012 - 16:45

If you are using silicone brake fluid, It might be a little "thicker" than the usual fluid and the booster might be moving too slow. This is usually not common in the XKE, but is common in the 150 with the leather bellows. If the booster was re-sleeved and re-built rather than a new unit, the brake shop might not have drilled out the tiny holes in the new sleeve properly. These are both rare problems, but enough to drive you crazy.

I agree with Bruce to bleed by pumping the pedal. Bleed slowly and watch the fluid at the bleeders for the bubbles to disappear. If tiny bubbles keep coming, then the master or booster have problems.

Phil.

Submitted by redbil@aol.com on Thu, 11/01/2012 - 13:52

Francois,

Quite puzzling. Any chance a bleeder is screwed into the bottom of a caliper rather than the top? When you replaced the booster, I assume that included the slave cylinder.

Good Luck,

Bill Braun