Intermittent braking problem

[marcus]

I've checked the manual, I've looked at Brunty's brake troubleshooting guide and I'm none the wiser, so I'm hoping what I describe below will ring some bells with someone...

Symptoms are:

Initial firm brake pedal feel and effective braking
Pedal then goes spongy and dives with resultant loss of braking
Pedal then firms up with effective braking but pedal then too close to the floor for comfort

These symptoms tend to show up a) when the engine is warm and b) under sustained braking as in slowing down coming up to traffic lights rather than when just applying a quick dab of the pedal to check one's speed.

These symptons tend not to occur when the engine is cold and at high idle ie with more vacuum available to the servo.

The car is a 1985 1.8 GTL, just had it's MOT, in the course of which one rear cylinder was changed by the garage, and the brake fluid was changed at my request.

This behaviour was happening before the MOT, and I had expected that replacing the fluid would have cured it (because the fluid hadn't been changed for years and I assumed it had absorbed a load of water and was boiling) - the cylinder change was unexpected - so I was surprised that it carried on after the MOT.

I am planning on changing the discs, drums, pads, shoes, remaining rear cylinder and hoses as a matter of course because the car is old and most of these parts have probably outlived their useful lives several times over, but would really like to know what the root of the problem might be.

I'm wondering if it's a problem with the master cylinder, servo or pressure regulator but it's pretty random, and I would have thought a problem in these areas would exhibit more consistent symptoms.

Help!

[scalainj]

Sounds like a loss of vacuum to me, perhaps a split hose then opens under pressure?

[go-for-it1]

Hi Marcus, this sounds very similar to the symptoms I had on an old Capri, after driving for a while the first time you press the brake it almost goes to the floor then if you lift off and push again it firms up? It was the brake master cylinder seals that failed. A new master cylinder and a good bleed carried out in the order given in Haynes manual and all was well but first I would check there is no brake fluid leaking at any of the wheels before spending on a master cylinder as they are not cheap. Another cause could be an old brake hose which goes weak and "balloons" as you increase the pressure in the brake fluid, difficult to confirm unless visible but new hoses are only about £7 so worth changing.
Rob

[celyth]

ive got the exact problem, a few m8s recon it's the master cylinder, i got a spare one here so gunna try that,

[Nate]

my vote goes on master cylinder too

[the edmundator]

As a point of detail, braking items such as the rear cylinder you mention really ought to be replaced in pairs. I don't think that's likely to be the cause of your problem though. I would agree with a master cylinder seal somewhere being broken as I had exactly the same symptoms when fluid was leaking from the brake light switch on the master cylinder in my camper. I had the added problem of brake fluid on the brake pedal... slippery!