Brakes holding on to pressure???

[Thunderbox1000]

Right - update from my other braking thread - master cylinder seals replaced, system bled and now a good brake!!! BUT - system holds pressure so after a few braking episodes the brakes bind and bleed nipples need easing off/on to release and get the pressure back to normal. Any ideas as to why this would happen??
I'm really keen to get his sorted so thanks in advance to all.

[piman]

Hello Thunderbox1000,

I had this on car once, but this was after overhauling the servos, I eventually found out that the vacuum piston seals supplied with the kit were too large and prevented the hydraulic piston from returning fully. Now if you haven't touched the servo it is more likely that your master cylinder is the cause. Either incorrect assembly of the seals, (do you have a manual?) or if there is an adjustment on the pushrod, this may needs slackening so there is a little free play?

Alec

[stuarthardy]

Try checking your servo reaction valve. It is easy to knock the top off and even easier to re-assemble incorrectly. If this happens then you will get the symptoms you describe. I know because I did it on my 420 a few years ago. Brakes wouldn't release.

Worth a look.

Regards

[PPV]

If the other comments do not solve the problem then I have had this before, but not on my Jag but on disc brake car. It was down to the flexibles to the brakes failing by going soft and expanding internally and acting like a one way valve. Ok to push pedal down but after a few uses the brake pistons would not release. You can check by squeezing them and seeing if they are 'squidgy'.
Worth a look.
Regards,
Paul

[sweller]

It was down to the flexibles to the brakes failing by going soft and expanding internally and acting like a one way valve.

I've had similar with the clutch hose on my S Type - it restricted the flow making it a very heavy pedal.

If the brakes drag on all four wheels it would point at the master cylinder and I'd doubt it's down to all three hoses failing together (possible, but unlikely).

If it is just one side on the front or the rears then the hoses would be chief suspect.

[jagman723]

Thunderbox

Have you solved this problem yet? I have same issue and am interested in this thread for obvious reasons.

Everything had been fine until I took car off road for some upgrades (around 2 years) then I got exactly this hard pedal binding problem when testing. Previously I had no problems with brakes, I'd had comments even that they were better than many modern cars. I replaced master cylinder, but still have fault. For me it usually happens after a few light/normal pedal presses or if I make one hard braking.

[Martec]

I've had something similar on two occasions, but two faults.

First one the brakes were perfect when pulling up but would not release for 5 seconds after releasing the pedal - I was using silicon brake fluid and the servo seals swelled and were slow to release.

Second, pulled up OK except for the last few yards when it juddered, and was sticking on when hot, using the MOT brake tester We found that a particular calliper was sticking on. On strip down I found the brake pistons were corroded and difficult to move, new pistons and seals fixed it.

Best of luck.

Brian

[Terlan1962]

Help!!!!
I'm new to MK2 ownership and I have the same issues as previously described (I know it's an old thread).
My rear brakes are sticking on so badly the car will not pull away an inch.
I have changed the master cylinder, which was badly corroded and not springing back out, but still the same issue.
If I release the pressure on the cylinder it frees up.
Could this be the hoses or is there a non return valve that I am unaware of?
All replies and suggestions gratefully received.
Thanks

[piman]

Hello Terlan,

the first place to look is the flexible hose, which can deteriorate internally and cause the symptom you describe. It will need replacing as it's not really possible to see any deterioration.
A sticking servo can cause similar problems but with all wheels not just the rear.

Alec

[jagjim]

I have the same hanging on problem ! I have replaced all the calliper pistons , the brake hoses twice , first time with Goodridge hoses , I was told these were the problem ! New master cylinder and all brake pipes ,all the servo hoses have been replaced and the chamber under the wing. The servo was replaced with new just before I bought the car, help . My car is a 1963 mk2 3.4 .

[Martec]

Hi,

Its always difficult to diagnose faults from odd comments or your intepretation of events, can I suggest you or your mechanic do a series of tests.

Jack the car up onto axle stands so the wheels are free of the ground and make sure the wheels turn freely.

Apply the hand brake and release to ensure it is free.

Apply the brakes hard and check to see which wheels are locked on. If all four wheels are locked on then it is the master cylinder or servo, if it is one front or both rear wheels then it is the hoses (I have had goodrich braided hoses on for over 16 years).

If they still don't lock on then start the engine to build up vacuum and apply the brakes again, if all wheels lock up then the air valve on the servo is at fault.

The servo seals are lockheed and will swell in silicon fluid, use only Dot 4 fluid, it is cheaper to change fluid to a good make than the cost of replacing parts.

Brake fluid is hydroscopic (absorbs water) which leads to corroded cylinders, and the chromed pistons seem to become corroded after 10 years or so, so the brake fluid MUST be changed every 2 years. The dirty dark colour gives it away.

Best of luck
Brian

[jagjim]

Sorted !!!! The brand new servo I was told had just been fitted was not new at all . I have replaced it and the brakes are now 100 % . I am poorer for believing what I was told , but that's life !