rear brakes locking on

[martinlaw]

Hi - I have a 1996 Sport and have noticed recently that the brakes occasionally donot release fully for a second or two later than they should (at that stage I was not sure whether it was the front or rear).

On Sunday I was conditioning some new front discs and the rear brakes locked on and would not release for about 3 or 4 mins.

Both back brakes locked at the same time and has done this several times now especially when braking hard.

I suspect it is the rear servo unit - anyone had a similar problem or can offer any suggestions?

Thanks.

[BigMack]

I would suspect the same thing. The calipers could need a thorough cleaning. Could be that the pistons are gunked up with dirt. I had this happen to me once, but it was due to not bleeding the rear brakes properly. They were hydraulic locking. You've bleed them I'm sure?
Cheers!
-Mack

[edfoxuk]

i'm aware of the occassional bind on my sport. All my calipers are fine/ Pedal travel is spot on, feel is good, very strange indeed!

[edfoxuk]

have consulted a local specialist who suspects it could be a master cylinder problem. Could be time for a seal kit or exchange units.

[GreenV8S]

Could be that the piston in the master cylinder is not returning fully when you take your foot off the pedal. This can happen if the pushrod is adjusted too tight (perhaps in an attempt to take all the 'slack' out of the pedal), of if the master cylinder is badly worn or gunged up. If this happens you may find that you can pursuade the master cylinder piston to come back slightly further by stamping the brakes and then side-stepping the pedal so that it flies back. If that works, it confirms that the problem is caused by piston travel in the master cylinder.

Edited to add: the last fraction of an inch of travel is vitally important, because it opens the brake circuit to the reservoir. Until this happens, the brake circuit is completely sealed which means that it can hold pressure, even if you have your foot off the pedal.

>> Edited by GreenV8S on Tuesday 11th April 17:53

[steve_D]

I agree with Peter (greenv8s) the piston is not returning.

Reference edfox. I would NEVER replace seals in a master cylinder. The 'cost' verses 'risk to life' equation does not stack up.

Steve

[martinlaw]

Thanks for all the advice.

I have now removed the master cylinder and will have it tested.

If it is ok, I can only assume that it must be the servo - this is going to be a real pain to get at as there are no access panels in my car. I assume the only way to remove it is to cut a hole in the front of the side pod?

[gtr-gaz]

I remember someone posting on here some time ago with the exact same problem.
I'm sure it turned out to be the servo. Have you tried a forum search?

[martinlaw]

Thanks Gary

Will have a look.

[GreenV8S]

martinlaw said:

I have now removed the master cylinder and will have it tested.

Remember that one possible cause is the pushrod adjustment, which would only be apparent when it was installed.

[martinlaw]

Peter - checked the freeplay in both rods (front and back) prior to removal - they both had around 5mm.

Could it be the cylinder in the servo sticking?

Cheers,
Martin.

[gtr-gaz]

found this. Could be some answers here?
www.pistonheads.com/gassing/topic.asp?t=150363&f=20&h=0&hw=servo

[martinlaw]

Thanks Gary,

That is really useful - will let you know what I find.

Martin.

[NDS]

I have tried to remove the servos on my 1998 Sports. It’s not a pleasant job! Or successful. I removed both the alloy kick panels and most of the floor. Unfortunately the only solution appears to involve removing the side pods.

Best of luck !

[Paul.B]

NDS said:

I have tried to remove the servos on my 1998 Sports. It’s not a pleasant job! Or successful. I removed both the alloy kick panels and most of the floor. Unfortunately the only solution appears to involve removing the side pods. Best of luck !

Can I suggest that if the side pods do need removing that you try cutting access holes in the underneath. They would need repairing but would be out of site. IIRC the Sport does not have the alloy inner pods therefore you may be able to access the servos through the bottom.

Just a thought! The idea of removing side pods on a built car ...... rather you than me!


Paul.B

[GTRMikie]

martinlaw said:

I suspect it is the rear servo unit - anyone had a similar problem or can offer any suggestions? Thanks.

When I first built my GTR I fitted the Ford brakes with twin servos. This set up worked reasonably well. When I upgraded to AP brakes, I left the servos connected. I then started to get the "lock on" effect you describe. I disconnected the servo vacuum hose from the manifold and the problem disappeared. OK, I no longer have vacuum assistance on the brakes, but this is not needed with the APs. Can you run your brakes without servos? They didn't appear to reduce the braking effort that much when I had the Ford setup.

[martinlaw]

Hi Mike,

Thanks for the input - interestingly my brakes are Willwood 4 pot ultralites on the front, the rears I cannot identify but they are 2 pot with a built in handbrake mechanism. This sticking problem has been around for a while. The master cylinder is in perfect working order, the vacuum hose appears to be ok so I can only conclude that it must be the servo unit.

I have ordered one from the factory and have now removed the old one. This I did this by cutting two holes in the front of the pod to allow access to the retaining bolts. The lower hole is larger than the top one to allow the unit to pass through.

I was not looking forward to doing this at all, but it was a lot easier than I antisipated.

Hopefully fitting the new servo in the next couple of days - will let you know if it cures the problem.

[steve_d]

martinlaw said:

.......This I did by cutting two holes in the front of the pod to allow access to the retaining bolts. The lower hole is larger than the top one to allow the unit to pass through......

To make good the hole in the front of the pod you could try a bulkhead hatch as use on sailing dinghys. It will seal the hole but still allow access in the future. It also gives access to some valuable storage space if you go off on camping trips.

Steve

[layada]

Due to the difficulty in removal I left the servos in place and just bypassed them (removed the banjo fittings) and put in a connector. Was a fiddly job but better than cutting the bodywork open. Solves the locking problem