21 replies to this topic

[Tobyrm1]

Hi,

Completely rebuilt the braking system on a 90's mini. Servo with the pressure compensator valve on the bulkhead.

All the components are new. The trouble I'm having is that the front brakes won't release freely. They bleed out fine and the pistons in the calipers are free when they are disconnected but cannot be pushed back when connected to the rest of the system.

Any ideas what could be causing this?

Thanks

[ukcooper]

callipers on the wrong side so they can not bleed propa

 ???

[Spider]

Check that the Master Cylinder is returning all the way. Don't just go off the pedal to check this, it could well be the pedal that's hanging up the master cylinder.

Edited by Moke Spider, 15 April 2017 - 08:42 PM.

[Tobyrm1]

callipers on the wrong side so they can not bleed propa
 ???

Definitely the correct way around, bleed nipple at the top.

Thanks

[Tobyrm1]

Check that the Master Cylinder is returning all the way. Don't just go off the pedal to check this, it could well be the pedal that's hanging up the master cylinder.

Will check tomorrow assuming it would be the Spring that returns the master cylinder?

Thanks

[panky]

If the air bleed valve is passing the servo will jam on with the engine running.

[Tobyrm1]

If the air bleed valve is passing the servo will jam on with the engine running.

I am not understanding what you are saying sorry

[Boycie]

I had this a few years back, although in my case the car had been sitting for a while.

 

The fault was in the master cylinder- something wasn't returning to it's rest position and therefore not allowing the fluid pressure to release the front brakes.

It was quite obvious what was at fault with the cylinder stripped down on the bench but I'm sorry, I can't remember exactly what it was.

A rebuilt master had it all working fine again. 

[tiger99]

As already said, the pedal must be hanging on the master cylinder, i.e. the pedal return spring (possibly broken?) is pulling the master cylinder push rod down hard so it is against its stop. On non-servo setups, special half shims are available, which can be inserted in pairs without undoing the clevis pin, to raise the master cylinder a bit.

 

If there is a servo, the same applies, sort of, and more so. The pedal return spring must be holding the push rod back off the servo input piston, and the servo output must have clearance between it and the master cylinder piston.And, there must not be an air or vacuum leak in the servo internals which is causing it to apply the brakes lightly, as this will result in the return port to the fluid reservoir being blocked.

 

A quick check. If non-servo, loosen both master cylinder bolts and pull it up a few mm. Do the brakes now release? If servo, loosen the servo and do the same. If that does not work, loosen master cylinder and do the same.

 

But it can be an internal master cylinder fault. The piston may be jamming because the seal lip has turned back and jammed between piston and cylinder wall, if the bore is worn or has been honed. Wheel cylinder hones, much used in certain parts of the world, must not be used on Mini brakes!

[johnR]

I had a servo that was binding internally - never was able to work out why but it seemed some expansion had occurred internally to the diaphragm plate if memory serves - ended up replacing the servo to save time but probably could have used the service kit.

[panky]

 

If the air bleed valve is passing the servo will jam on with the engine running.

I am not understanding what you are saying sorry

 

The servo air bleed valve is opened when the brakes are applied, if the valve is faulty, or stuck open, then the brakes will come on by themselves - but the engine has to be running for this to happen. Just another possibility to explore,.

[minidaves]

the drivers side brass conector with banjo bolt, does it have a copper washer on both sides? if not this would slow the flow down. and when you say does not release, you cant turn the wheels or it just rubs?

[Stu1961]

Morning all,

 

Reading this with interest as I'am currently re-building my braking system and have added a split system servo (82 Mayfair) could the issue be a weak or broken servo return spring as the pedal on a servo car lacks its own return spring relying solely on the servo to return the pedal to the rest position I think (I please put me right if I'm wrong) If this is the case surely the pedal will hang on the servo so not completely releasing the brakes.    

[Tobyrm1]

Still the same with the engine off.

They release slightly when you press the brake but rub quite a lot. Definitely wouldn't want them any tighter. In two minds whether to investigate more or see how it all beds in. What concerned me was that it impossible to push the pistons back completely without disconnecting.

The brass 'T' piece definitely has two copper washers too. Going out to
Investigate further now!

[Tobyrm1]

Just checked the brake pedal, definitely returning to the off position with the Spring pulling it back.

The front wheels will move easily by hand but won't free wheel.