rear brakes lock up

[flying]

hi people....my sister seems to be having alot of problems with the rear brakes locking up...when it was under the restro i put new seals in the master cylinder,its had new brake pistons,brake pipes,new handbrake cable etc and yesterday i replaced the shoes and put new drums on so they haven't been fully bedded in yet,....so pretty much everything ,they are adjusted right but it just keeps happening ,also notice that the rear brake lights keep coming on when it happens, sometimes they come on while reversing it had new brake switch has well, .....has any one had this problem before or maybe know the cause of the problem....has i cant think why it keeps doing it ..... any ideas

[Willie]

If the brake lights come on when this lock up occurs then it surely suggests that there is pressure in the system which is causing the problem. This raises the question as to why the front brakes don't lock up at the same time (presumably) so, what is the condition of the rear flexible brake hose? Does it only happen after use of the foot brake,
or does operation of the hand brake also give this problem?

[flying]

the front brakes don't seize up,the front flexi hoses were replaced but not back one....do you think that could be suspect....prob only thing that hasn't been replaced on the braking system .it tends to happen after she drives it for a while or a few miles ....seems to happen has if they are getting hot and expanding, handbrake is lose enough when released and wheels do turn when i try also with adjustment.....if i adjust the shoes right down has far has i can go the the foot brake doesn't apply no pressure to stop the wheels so i set it up so its just right so wheels can still move freely and can apply the pressure

[bmcecosse]

Sounds like master cylinder not correctly adjusted for stroke - not returning properly. Brake light on means pressure (as above) - and so it's not just a flexi. Or maybe if there is a servo involved - could be servo problem ?

[flying]

doesn't have a servo, but it does brake quite well for drums on a non servo ,the brake light only comes on after it seizes on if you leave it 10-20 mins seems to free its self app.....the 1st time it happened she broke down on a busy road took me about 5 mins to get to her and when i tryed it they were fully stuck on....only way i got it off the busy road was by giving it some revs and managed to prize the drums off then they were free again for it to be drivable but since then ive put new drums and shoes on

[bmcecosse]

Must be pressure - you could have eased a bleed screw to release the pressure. Try adjusting the master cylinder stroke.

[WPR678B]

As BMC says, the master cylinder stroke sounds to be wrong. In the chassis rail attached to the brake pedal you have a rod that is connected to the brake pedal with an adjuster on it. This needs to have at least 10 - 12 mm of movement (at the top of the pedal) before it contacts the piston of the master cylinder. I learnt this from experience after a rebuild as i had very similar problems!

[flying]

when i did the master cylinder i stuck the adjuster out has far out has poss to give it more pedal movement...if i pull it out any further it will fall out

[alex_holden]

Don't you screw the adjuster in (thus shortening the piston rod) to increase the amount of free pedal movement before the cylinder starts to act?

[flying]

i would have thought out ,the bar pushes the piston and then makes the pressure by pushing the piston towards the back,if the bar was pushed inwards i would have thought that it would be applying the pressure rather than having less pressure i made a little sketch sorry not that great but hopefully you will see what i mean by the theory of it

[alex_holden]

When you screw the yoke into the push rod the combination gets shorter.

[flying]

yes it does ..... .....ill have to check it....well ill do that and see what happens ,thanks

[bmcecosse]

It must have some free play - and a return spring of course.

[Willie]

If the front brakes do not also lock when the rears do then logically it is
not a system pressure problem just a rear fault. Re the pedal free movement adjustment A WARNING. The free play must be adjusted so that you will have the necessary free play when the floor plate is IN POSITION. If you do it before the plate is screwed
back on then it will be wrong and there will be no free playleading to the build up of pressure in the whole system. So keep checking by returning the plate to its position whilst you are adjusting. I mentioned the rear flexi
hose as it is possible to have one which is collapsing so that the pressure from the brake pedal is enough to force fluid through but the brake springs are not strong enough to return it. Unlikely but possible. Also, there is not a lot of thread on the operating rod so if you unscrew it from the Yoke to decrease free play at the pedal you are perilously close to it falling out if the play is caused by other items such as worn bushes on the brake pedal etc. When I say free play I mean the tiny amount,with the floor plate fitted, before you feel the rod contact the piston. I have
1/16"on my car.

[bmcecosse]

The brake lights are said to be staying on - they are operated by a pressure switch - ergo - it's pressure! Perhaqps the front brake return springs are just enough to let them go round - but the rears are not.

[flying]

well i had a look and adjusted it and yes the pedal goes further down now,but then dont get pressure on the rear brakes and if i adjust the rear brakes another click they dont turn easy......i only had 1/2 hr to have a fiddle has my sister has weird work hours and she needs the car for work....at the moment the back brakes dont really work if the brakes are applied , so will find if they still stick on ....will have another fiddle when i can have the car a bit longer to adjust things....but hopefully the rod adjustment is the problem

[alex_holden]

Do you mean the pedal is now going right to the floor without locking the rear brakes? What are the front brakes doing?

[flying]

before the pedal was quite stiff ....really stiff with hardly no movement....now it has movement in the pedal which is better than before....it doesn't go right down to the bottom.....didn't have a look at the fronts but they do stop the car well if you push the pedal so its better than before

[Willie]

if the front brakes work for a given pedal movement then the rear should do so as well. If this fault appeared after work was done on the
Master cylinder I wonder if you have problems with the rear brake pipe
shape? if it had the wrong shape it would be closed up as the union was tightened so restricting flow through the pipe. This would be more likely if you had fitted new pipes during the overhaul. You already know that application of the foot brake doesn't operate the rears as it should so
I would suggest that when they lock up again you immediately slacken one of the rear bleed nipples, if this releases them then I would suspect
the union as above. The same fault could apply to the three way union on the rear axle! Brake pipes come with two different type of end, flared or domed, if you fit a flared end to a joint where it should a domed type it will close up with the tightening of the nut.

[bmcecosse]

How do you know it's not working the rear brakes ? Is it just that they don't lock and slide on the road during an 'emergency' stop ? May be worth asking local garage to give it a few minutes on the MOT brake test machine to see exactly what the efficiencies are at each wheel.