6 replies to this topic

[Dartmoor Paul]

My son has rebuilt a 1975 Mini Clubman.  He has fitted Metro 4 pot calipers with a bias valve fitted on the rear sub-frame.  Otherwise the standard single brake line configuration has been retained.  The only other modification has been to install a remote brake servo.  This servo was completely overhauled using a genuine service kit.  Everything appears to work perfectly except when slowing down.  When slowing down to a stop, the servo is applying the brakes without the brake pedal being depressed.  The brakes release again only when the accelerator pedal is pressed.  The servo does not lock the brakes, but merely causes them to bind enough to prevent any free-wheeling.

 

All of the hoses are new and as I say all of the parts were replaced using a genuine service kit.  I've checked that the non-return valve in the vacuum hose connecting the manifold to the servo is the correct way round.  The only part which is not correct, is the adaptor to connect the vacuum hose from the manifold to the servo.  We managed to obtain a similar part from an engineer.  We could not locate the correct item.

 

Can anybody help please?

[Spider]

First thing I'd check is that the Master Cylinder is completely returning, the pedal should 'hang' off the Master Cylinder when at rest. The other thing is the Master Cylinder has (or should have) a Residule Pressure Valve within it, if this is maintaing too high a pressure in the system, it could also behave in a way that you've described, though these usually bleed back over about 1 - 2 minutes.

 

However, given what you've described, in particular that the brakes start coming on when you decellerate and come off when accellerating, this also coresponds to the Vacuum that would be in the Manifold (and in the booster). It sounds very much to me there is a fault (sticking?) in the Contol Valve, which is part of the Booster.

[nicklouse]

Sounds like you need to bleed the brakes or you have something not set right as a servo is just a means to add force not create force.

So either some how the master cyclinder is applying pressure to the servo. Or the braking feel has nothing to do with the servo.

I would first disconnect the servo pipe and block the inlet port and see how things are with no sevruga help.

Still dragging? Time to check the brakes.

[peter-b]

My money would be on the remote servo. Sounds like the reaction valve is either assembled wrong or faulty. As mentioned, clamp off or block vacume inlet, if all is fine, albeit no assistance, take the booster back to who ever o/h it for you.

[Dartmoor Paul]

I probably didn't explain the problem correctly.  The brakes do not apply themselves, instead they do not release fully until you blip the accelerator.  That said, you've provided some really useful advice.  I'll give it a try and hopefully get it fixed.  I must admit, that I tend to suspect the servo rather than the master cylinder.

[KernowCooper]

Servo as its a remote type like mine there is no adjustment between master cyl and serve

[Dr s]

If you just disconnect the servo and block up the inlet pipe what happens? Less brakes but do they behave?