5 replies to this topic

[WillS66]

Hi,

 

I have an '89 Mini Racing (998cc) with the John Cooper conversion kit offered at the time (twin SU carbs). The car has had a lot of cosmetic modifications but mechanically it's pretty much the same as it has always been.

 

A few days ago I noticed that on depressing the brake pedal, the engine revs increased substantially. The brake pedal also seemed very firm and brake performance poor.

 

Had a poke around under the bonnet and noticed that the hose between the brake servo and the manifold seemed to be working lose at the manifold end, so I tightened it up.

 

Now when I press the brake pedal, the revs drop and the engine stalls. The brake pedal still seems overly firm and I suspect that brake performance is as poor as it was when the problem was noticed (I haven't driven the car since noticing the original problem).

 

I've tried starting the engine with the brake pedal depressed and the pedal doesn't "drop" a little as I'd expect it to. There's no obvious hissing from any components.

 

My assumption at this stage is that I have a problem with the hose, the non-return valve or the servo itself (or any combination of those three items).

 

Before I start spending money on replacement components has anyone any ideas on narrowing down the problem? Or any suggestions for the root of the problem that I haven't considered?

 

Thanks

 

Will.

[nicklouse]

servo needs sorting.

[Ethel]

Removing the hose at the servo end and blocking it would narrow the problem down to the servo if it cures the engine faults. Obviously it would also reduce the brake performance.

 

An explanation you might not need, but it is useful for the logic of identifying the fault:

 

The servo has a diaphragm inside, dividing the servo can in two with a simple communication valve at its centre that's operated by the push rod from the pedal.

 

At rest both sides are connected to the manifold vacuum by the hose. There's a non return valve in the hose connection, so the servo can maintain the lowest pressure the manifold reaches, even if the engine stops.

 

When you press the  brake pedal the valve in the servo simultaneously  opens the rear half to atmospheric pressure and seals off the communication across the diaphragm to the master cylinder side. It's the pressure difference across the diaphragm that increases the effort applied to the brake pedal.

 

So, it sounds like the valve is opening to atmosphere correctly, but the diaphragm isn't sealing off the front chamber when it does allowing air to flow in to the manifold and weaken the fuel/air mixture. Either the valve is faulty or the diaphragm has a hole in it.

[WillS66]

Thanks for the advice, I'll remove the hose at the servo end tonight to see if that narrows it down a little.

 

Additionally I've been thinking about the timing of this fault. It happened on the first drive since I replaced the front, driver's side rubber suspension cone. I wonder if I may have knocked or damaged something while removing the bolt securing the subframe tower or using the cone compressor tool. It's pretty close to the servo and I'm suspicious of coincidences.

 

Will.

[WillS66]

Removing the hose at the servo end and blocking it would narrow the problem down to the servo if it cures the engine faults. Obviously it would also reduce the brake performance.

 

Removed the hose last night, blocked the end, and the engine faults disappeared. With the hose removed I took a quick look through the hole into the servo and from what I can see the diaphragm is completely perished. It looks like an empty drum in there!

 

Replacement unit on the way. Thanks for the advice and the useful explanation.

 

Will.

[Ethel]

There is a repair kit for about half the price.

I know I only mentioned the diaphragm but there's also a sort of pressed steel "piston" you might have been looking at that.