Brake pressure loss

[jonbowen]

Strange things are afoot here! Apparently simple problem: every 4th or 5th push on the brake pedal and suddenly the pedal gives way like one of the brake circuits has failed completely ... starts to push after about 1/2 way down, but braking much reduced. Doesn't hit the floor though, unless completely stamp on the pedal. Pump the pedal a few times, and normal functioning is restored ... for a while.
This doesn't happen when the van is stationary. There is no loss of brake fluid from the reservoir, I've checked it every day for a week, and there's no obvious signs of leaking pipes. The problem started after I took it to a mechanic to replace front offside wheel bearing. I thought he must have disconnected the brakes for some reason, and then not bled them properly, so I took it back and asked him to bleed the brakes properly. He pleaded that he hadn't touched the brakes, but bled them anyway. No improvement, so he said it must be the master cylinder, which kind of makes sense.
So, I got a brand new master cylinder from GSF, and since I hate bleeding brakes, took it back to the same guy who fitted it. Still no improvement, then he found the rear brake shoes v. worn, so he replaced those, though I can't see how that would cause the problem.
He seems to have used about 3 gals of brake fluid bleeding the system, but it's still exactly the same. I've paid this guy £300 now, and put in quite a bit of time myself. Short of stripping out the entire brake system and rebuilding it, don't know what to do. Anybody else had a similar problem? Any ideas? The only thing I can think of is that the new master cylinder was faulty in exactly the same way as the old one ... not v. likely.
Can't see that it would be a slave cylinder problem, as presumably that would either lead to fluid loss or the brakes pulling to one side? The only other moving part in the system is the pressure compensator valve, but can't imagine how that would cause these symptoms, or even that such a simple component would fail at all ...

[peasant]

Any chance that one of your brakes is sticking?

Overheating of the brake would cause the brake fluid to bubble and lose pressure. Have you checked rim temperatures all round after a bit of a drive / once the phenomenon happens ? If one of the brakes were getting that hot, you'd spot it immidiately.

[steve8090]

It is quite common for the top plate with the adjuster wheel of the rear brakes to be fitted the wrong way round thus holding the shoes away from the wheel cylinder, the long prong should be at the back, if its fitted to the front it holds the handbrake cam away and gives false adjustment, even with the brakes adjusted correctly there is some "slack" between the cylinder face and the shoe face causing exactly this problem also badly adjusted rear brakes would also cause this problem

[jonbowen]

Thanks for the ideas - Sticking brake definitely not the problem - sometimes it happens just 10 seconds after setting off in the van in the morning - 20 yds down the road.
Rear brake adjustment could well be the problem, I had no idea this was so critical, or that things could be mounted the wrong way round! I'll print out your comments and take it to the mech. ... after all, he has just been messing with the rear brakes! I guess things could have got out of kilter with the rear brake shoes been worn away to almost nothing, and then the mech. didn't put it all back together right - he *was* in a real hurry.
Thanks again Steve & Peasant,
Best wishes, Jon

[jonbowen]

I've now had the back wheels off and had a good look. The adjusters are definitely mounted properly, and I've tightened up the adjustment - probably more than I should. Oddly, it's much more reliable when running now, but the first time I use the foot brake after the van's been standing for 15 minutes or more, it's always really bad ... because we're bang on a main road, this can be a mile or two up the road, so I hadn't clocked that it's particularly bad when starting out.
The way I figure it: Either the master cylinder is faulty or not.
If it isn't faulty, and is pumping brake fluid into the system, that fluid must be going somewhere:
a) compressing an air bubble
b) taking up slack in the brake cylinders/brake pads
c) pissing out of a hole
Can't be (a) because the system's been bled to death; can't be (b) because I've checked the rear brakes and the mechanic has checked both front and rear; can't be (c) because the fluid level in the reservoir isn't diminishing.

So the master cylinder must be faulty despite being brand new.
Can anyone fault my logic?
Is it possible there's an air bubble stuck in the system somewhere that just won't come out?
Best wishes, Jon

[getunder]

I once had a mystery loss of pressure on a rear caliper of a Rover 2000. It could be bled hard in the evening and sometimes the pedal would go to the floor in the morning. Somehow the caliper let back in air but did not let out liquid even under high and continued pedal pressure. Nobody in the entire Rover 2000 Club even pro mechanics had ever heard of it-but it was a fact. Oddball things can happen. Leave no stone unturned.

[getunder]

If problem persists can you make up a piece of metal brake pipe with a union on it and hammer the pipe flat so it does not leak and use it to isolate parts of the system in turn including master cylinder. This can also be used in normal brake dismantling to prevent loss of fluid in some circumstances.

[Oldiebut goodie]

Stab in the dark here.
You haven't got a hose ballooning under pressure?

[billy739]

make sure front wheel bearings are adjusted properly

[Gunslinger]

Sounds like the wrong hub bearing has been fitted. I had the exact same issue and once the bearing had been replaced with the correct one all was ok again.
From memory the issue is the internal bearing dimension on the bearing is slightly larger than the stub which means the disc arrangement doesnt sit perfectly square.
You will find that if you leave it like this the bearing will collapse fairly quickly.

[jonbowen]

Thanks everybody for your contributions - And the Winner Is: Gunslinger with the Wrong hub bearing!! All fixed and working now, just got to get my money back from the idiot mechanic who fitted the wrong bearing in the first place. I would have forgiven him if the last thing I'd said to him wasn't: "There are 2 bearings for the T25, it might be the wrong one, can you check carefully before you fit it .... ".
Best wishes, Jon Bowen