Help! brakes being weird..

Hi all.

I have a disco 300tdi 97. The brakes randomly come on by them selves and stay on... until cooled down (about an hour) then free them selves up. this has been ongoing, i've changed flexi hoses and caliper. it's worse on near side front (that one brekes harder) the motor does not have ABS..

Any ideas?

Could be a servo master cyl problem.

As far as i know, not much else left it could be... can anyone tell me the function of the vacuum pump? I can't see how the master cylinder could initiate braking by it's self...?

vac pump creates vac for servo at all times ,pressing pedal and moving diaphragm allows vac to assist pedal ,remove pipe from pump or servo and try brake pedal will be harder but will still work ,servo is assist ,it could be poss that servo was using vac when not needed but never come across it usually siezed pistons or warped discs or wheelbrgs

when brake comes on crack bleeder. This will establish if it is a fluid induced fault or not.

Nice one cheers fellas. I'll update when something new to say..
It's just so bloody random! although it's done it twice in the same time/place.. coming back from a friends early morn, about 8 mile no trafic no braking then just over the brow of the last hill on they come... same place twice...????

Oh yeah and I've put new pads and discs front and back + nsf caliper and braided flexi hoses all round.

and your still driveing it on the public highway

Yeah. they don't lock up you just start slowing down... at which point i pulls over and walk. inconvenient but not particularly dangerous.

Do you have the problem still ?

I'm new to the forum / Disco ownership - but I worked for years on brake system design & have seen this problem a few times.
Have you checked whether the problem can be removed by cracking a bleedscrew - as suggested by fyne1 ?

When it happens, you need to jack up the worst offending axle & confirm if the brakes drag badly or are siezed.
If yes, crack the bleedscrew & if the problem went away, you know its a problem up at the brake mastercylinder - which should not normally allow pressure to be trapped in the system. The brake dragging problem will happen because when the brakes get hot, the brake pads & fluid expand. If the expansion can not happen because the mastercylinder is sealed even when the brakes are not applied, it creates pressure in the system - automatically applying the brakes. If you continue driving, it can get to the point where the engine is not powerful enough to overcome the drag (and the brakes can actually catch fire !)

If the problem didn't go way after cracking the bleedscrew, check the pads to make sure they can slide in the caliper housing. You should be able to push them / the caliper pistons away from the disc.

If pressure was trapped / released by cracking a bleedscrew, there are 2 probable causes :
1 - the mastercylinder pistons are held forward of their "proper" position or
2 - the mastercylinder seals are "swollen" due to contamination in the fluid.

To check for "1" or "2", try pushing the pistons back into the calipers. Because the system is sealed, the pistons should not move inwards very far.
Now undo, the mastercylinder to servo retaining nuts & try to push the caliper pistons in again.
If the pistons will go all the way in, separating the servo & mastercylinder has allowed the piston to return further back to its "correct" position and reduced the pressure.
This indicates that the servo output rod is being held too far forward - either because it was not set correctly when new (unlikely since your vehicle is already 13 years old & it would have been spotted / sorted by now !)
OR because the brake pedal / pedal box is causing the servo inputrod to be held forward.
If this seems to be the cause, have a look at the pedal mechanism for problems which prevent it from returning fully.
If separating the booster & mastercylinder didn't allow the pistons to be pushed in, you probably need a new mastercylinder – as the seals can not be changed easily.

Hope this helps !

Paul

Thanks a lot for all that Paul, superb bit of info.

As an afterthought, I should have also asked the standard questions of when did the problem start & what was changed prior to the problem starting ?
(From 20+ yrs of experience, almost no brake problem ever started "out of the blue")

Paul

It first did it about 6 months after getting the bus, (had changed nothing) was told prob the flexi hoses so changed them when i put a lift kit on. It didn't do it again for about another 4 months. since then it can do it twice in a week then not for months... random.