Brakes locking on

[Dave2000]

The 1500 has developed an annoying habit of locking its brakes on  :-/

It did it once before, but just on the OSF, and so I assumed it was the flexi-hose on that side, and so changed them both. Then it seemed to go OK for a couple of months
On Saturday, we were out in and happened to come down Fish Hill. A couple of miles later you could feel it struggling, and smell heated brake pads.
Pulled over, and both front brakes were merrily smoking away, but worse on the NSF (rears felt a bit warm too) Just after I finished calling the AA they freed off again....
What would cause this? My initial feeling is that the master cylinders' worn internally, but could it be anything else?

Need to sort this quickly as it's Jemma's only form of transport   ??)

[Bob Bowling]

Fronts: could be sticky calipers - are the pistons free moving or a real effort to push back?
Rears: check the pull off springs, the leading shoe is pulled against the drum in normal rotation and the spring has to keep it off. Had this once on a Herald and new springs cured it completely.

[GT6 M]

the fronts,  
1, rusty pistons/ surrounds, not letting the pistons float
2, damaged seals,  holding muck in
3, worn seals,  its only the deformation of the rubber, that ..pulls.. the pistons back

back,

1, stickin piston
2, broken spring / s
3, hand brake mechanism sticking on / not releasing
4, build up of brake dust  / crap

Matter of elimination  ole boy,
regards Marcus

[partsaver]

Is the peddle returning fully?

check the pivot bush on the peddle assembly and the return spring.

[Dave2000]

OK, thanks for the answers so far. To answer some of the questions:

The pistons felt free enough when I changed the pads a couple of months ago when it first started playing up. There may have been a small amount of corrosion around them, and one rubber seal looked slightly damaged, but not enough to flag-up as a problem as they all moved in and out OK

The pedal returns to its normal position no problem, and stays there Ok even if the brakes are locked on. It just goes hard and doesn't have much travel when you try to press it down

If it were the springs on the rears that had gone weak, why would that lock the front ones on? I can understand that the rears would drag, but not why the fronts would lock on enough to make the discs smoke through overheating

[heraldcoupe]

It's probably the main spring in the master cylinder. They work harden at a point about 1/3 along their length. It will either have physically broken at this point, or if it's not reached that point, it will simply fail to compress in this area.

This problem is sometimes brought about when the pedal return spring breaks, leaving internal spring to do all the work.

Cheers,
Bill.

[piman]

Hello Dave,

it it has a servo, the main air piston could be sticking? I had that experience before.

Alec

[Mj17]

I've got a similar problem on my car - the brakes don't seem to fully release (though not to Dave2000's extent).

In my case press the pedal and it's firm from the very top.  Take your foot off and press again and there is no resistance at all for the first part of the travel (how much varies), then goes firm like it did at the top of the pedal first time.  Front pistons all seem to move fine.

To try and fix this I've:
- Replaced servo with a new one.
- Removed servo completely.
- Replaced the rear cylinders with new.
- Replaced the replacement rear cylinders with new.
- Replaced coper brake pipes with new.
- Replaced master cylinder with new.
- Filled wear in p/s rear back plate due to and replace handbrake actuating arm.

Currently 4x new braided, flexible hoses are on order - to replace the ~15 year old braided, flexible hoses and I'm pricing recon. callipers - to replace ~10 year old recon. callipers  :'(

[alan pettit]

i had brake problems like this with an old jensen healey after driving for a few miles. it turned out the inner bore of the flexible hose was broken with a peice hanging off internally  and was preventing free movement backwards of the fluid. It wasnt leaking as such as the fault was on the inner bore not the outside casing.

[partsaver]

If it was the flex pipe then it would only effect that brake, i.e. one for the rears or one each side on the fronts, from your latest decription it does sound like the m/ cylinder or where the pipes from the m/c join on the bukhead there is a valve  if you have  duel crcuit brake lines fitted but if this is faulty it would normally just stop the brakes working.

[Pete Lewis]

if its pressure building up then the recuperation seal in the end of the master cyl is not releasing with foot off,
or there is no free play in the push rod due to brake switch mis adjustment or seized pivots

iregular drop in pedal height after after use is often too much free play in front wheel bearings

hanging on can be as said handbrake cable holding shoes open and servo  pushrod or airvalve stiff.

is it fitted with a pressure diferential valve is it seized ?
could even be a seriously contaminated Mcyl seal, thats grown

Pete