Brakes require pumping!

So this is a struggle.

I fixed the three amigos around 10 years ago with the bypass mod, but it went wrong again, this time the error was a pump fault, and sending the ABS modulator away confirmed it was faulty and non-repairable.

So I got a fully reconditioned unit, fitted that - sorted the problem, but I now found the brake pedal would slowly go to the floor.

So replaced the master cylinder with a new one, which has sort of fixed it.

However there is a new problem, press the brake pedal and it goes almost to the floor, pump it once or twice and it goes solid (well defender solid anyhow).

Sit there several seconds to a minute and again the cycle repeats - pedal goes to the floor, one or two pumps and back solid.

Importantly, the vehicle is not moving, just stationary whilst we find the problem. A few years back it had stainless braided hoses fitted all round, it had the front pipes replaced and new pipes all around at the calipers.

It has been pressure blead (several times now) including power blead through the modulator, no leaks anywhere.

I can't find how to solve this at all, the master cylinder has been changed, no driving, so not wheel bearings, when it goes solid, it's more solid than ever before, all new brake fluid, no air bubbles and the fluid isn't going down .

Totally stuck

Any ideas guys?

Have you a fluid leak in brake servo ?

Do you mean an air/vacuum leak?

The servo is definitely holding the vacuum.

WhyIsThisSoHard said:

Do you mean an air/vacuum leak?

The servo is definitely holding the vacuum.

Click to expand...

Master cylinder leaking into the servo ?

Or even seal fail in master allowing fluid to flow around inside it.
Even if new I am afraid to say quality of some parts is poor these days.

Anaconda said:

Master cylinder leaking into the servo ?

Click to expand...

No, it's clean

Also no drop in fluid level

tottot said:

Or even seal fail in master allowing fluid to flow around inside it.
Even if new I am afraid to say quality of some parts is poor these days.

Click to expand...

Well the new MC stopped the sinking of the pedal, so I don't think it's that; but I think that was the issue previously.

Forgot to add, the replacement MC is a delphi one, so not really cheap ebay rubbish , but not the best of course.

OK, found the problem!

In the brake servo, there is the plunger/rod that goes into the master cylinder - it is this that drives the piston.

It's held in place by a star washer/clip - turns out this was loose - so as the engine runs, the vibration causes the plunger/rod to shift about pushing the master cylinder piston down a little, leaving a gap between the plunger/rod and piston or diaphragm - thus after a bit when you press the brake, it has to take up this gap before it starts pushing the piston.

I used a bit of tube, pushed the washer/clip hard into place, with the plunger/rod now fixed to the diaphragm it doesn't produce any gap.

Took some finding!

Well done on finding the problem and reporting back.