brakes siezed on - brakes siezed

G

gary smith

New Member
Hi everyone I have just used a repair kit in the brake master cylinder and renewed the brake fluid,the car had new front flexi pipe a couple of years ago,when bleeding the brakes at the front after a few pumps the pedal wouldnt go down unless the bleed niple was undone,which I thought nothing of.Took the car for a road test all fine and dandy pedal seemed a bit hard but I thought must be the naster cylinder working better,then it happened front brakes seized on solid limped home (about 2 miles) in 1st gear stalled when in 2nd! the pedal is rock hard and even after leaving for 3 hours still siezed solid! any ideas please as I am losing faith very quickly.There was no siezing before I replaced the master cylinder rubbers,just a crap pedal pressure. cheers gary.
 
Sounds like you might have reassembled the master cylinder incorrectly OR more likely (hopefully!) have not adjusted operating rod correctly. The master cylinder piston contains a valve that allows the fluid to flow back into the resovoir when the brake pedal is released. If the acutuating rod is adjusted so that it is applying pressure to the master cylinder it won't allow the valve to open and the brakes will lock on once applied.

Try backing the rod off completely to see if the brakes release before any major dismantling.
 
Hi I have backed off the operating rod to the brake master cylinder no change brakes still stuck on,I pulled off the flexi pipe at the servo and within 5 minutes the brakes where free.any ideas anyone please.cheers gary.
 
Can anyone advise me before I start laying out loads of dosh if my problem could be the servo? should I buy a repair kit or could it just need a bit of lubrication inside? never had problems with rover servo,s before or on any car I,ve owned so totaly in the dark here.The car has only done 500 miles in the last 11 years so I,m now coming across all sorts of problems associated with lack of use! cheers gary.
 
It could still be the master cylinder. If it isn't releasing it won't allow the reaction piston in the servo to retract so the servo will be locked on. Could be a very stiff reaction piston or a dodgy air valve but the brakes were working Ok before you serviced on the master cylinder, weren't they?

If it was my car the first thing i'd do now the brakes have released is jack up the front and take off the road wheels. Try pushing the caliper pistons back in and check fluid retruns to the resovoir. Then see what happens when you apply the brakes, do they sieze on again? If not then try with the engine running and the servo pipe reconnected to see if it's related to the servo.

Bennet
 
Thanks bennet done that today the brakes dont sieze on,but when I reconnect the servo pipe and start the engine the pedal goes rock hard as soon as I press it and the brakes sieze on again. cheers gary
 
Sounds like the servo air valve isn't releasing for some reason. Might be an idea to check the reaction piston first.

Take the cover of the air valve (white plastic cover with 5/6 screws on top of servo) Pull of the diaphragm and you'll see the small reaction piston in the bottom. Try pushing it back in with a small screwdriver and then get an assistant to push the brake pedal to see if it comes back up the bore again. Should be reasonably free if you work it back and forwards a few times but has a seal on it so some resistance will be felt. If it is very stiff you can take the air valve base plate off and carefully eject the piston using the brake pedal. Clean it and put some silicon brake grease on it.

If thats OK and then it may be the air valve is kaput. Most suppliers sell the air valve as a replaceemtn part for about £30 ish. (I've just changed mine) Although it doesn't look like much it's quite a sensitive device and if the rubber seals that control the vacuum are suspect it can lock on.

You still haven't said whether the brakes were working correctly before you started work.
 
Hi before I replaced the master cylinder rubbers the only problem with the brakes was that the pedal would be okay one minute then when pressed again the pedal would go practicaly to the floor when this happened the brake pedal would have to be pumped twice to stop the car.I have just had a run up the road in the car without the servo pipe connected,and no sign of brakes siezing at all,in fact the pedal felt very good considering no servo connected.Thanks a lot for that info on the servo,going on holiday tommorow so will have to wait till I come back,thanks again gary.
 
Hi everyone just got back from hols yesterday,never seen one p6 in italy plenty of fiat 500,s and old merc,s. Started investigating the brake problem again today,took off the air valve and noticed bits of black plastic on top of the diaphram,then took off the diaphram and checked the neccesary as advised everything working okay.On further inspection of the air valve there seemed to be something missing there was a spring loaded piece of plastic that sat on the diaphram but it seemed to be too short to do anything,looked in the manual and lo and behold there was a black plastic cap missing hence the black plastic bits! I found an old servo I didnt know I had took the air valve off and there it was complete with the black cap etc.fitted this 20 mile road test and excellent everything worked as it should.many thanks to all the advice from members who steered me the right way to solve the problem,and here,s me thinking of accepting an offer of a swap for a morris minor in despair.cheers again gary.
 
Hi all,

just finished rebuilding the servo and master cylinder in my 2000 TC. It was needing it badly as the pedal had to be pumped a few times to built up pressure, and the servo was full with fluid.

Everything went well, cleaning, painting and assembling everything back using new seals.
Even my first attempt on bleeding the brakes on a P6 was pretty straightforward. The rear bleed nipple was easy to get to after raising the back of the car, and an one-man-bleeding-kit found its way into it. After the bleeding everything felt as it should, but lets start the engine to see what happens.
A whistling noise and brakes locked after the first pedal push was the result. Something was clearly wrong with the servo...

I released the pressure in the system by undoing a union after the servo. The whistling noise and vacuum leak was traced down to a bent air valve base plate. I replaced it with another and it was slightly better but still leaked. The air valve piston is not sticking into its bore and everything else seems to assembled as per workshop manual.
So, could the locking brakes be due to the leak in the air valve?
Can i use some gasket sealant to the cover of the air valve?
 
DaveHerns said:
Have you got sufficient free play in the pushrod into the master cylinder ?
Click to expand...
I had a quick look today, the pedal was indeed out of adjustment, but after adjusting it according to the manual, we had no difference. I managed to seal the airvalve properly so now it doesn't leak. If i undo a union before the servo the pressure on the system is released, which means that the servo will allow the fluid to return, but not the master cylinder.
i must have done something wrong during the assembly of the master cylinder then. Even if it looked quite straightforward. Mind you, what i only did was to renew the seals on it. I hate to take it apart again, but i am afraid that i cannot avoid it.
 
A weak return spring on the pedal means that sometimes the pedal does not fully return to the highest position.
 
FINALLY :D

I have brakes that not only grip tightly the discs, but also they release fully :)

As expected it was something rather simple. The master cylinder rod retaining washer was fitted the other way around, thus not allowing the rod and piston to retract fully.

When i looked into it was obvious, but it was something i didn't notice in the first time. Well, this what happens if you work in your car in the night :;):

After all i was used of the master cylinder design in my Austin which does not have such details.

I have to keep in mind that while rebuilding this car, i am learning it too, all at the same time. And i must say that i appreciate the way it was built. You just have to understand first how something works, before attempting to take it appart :;):
 
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