strange brake problem

[renegade53]

here's one for you, the suggestions i made don't seem to have sorted the problem. a mate has about 7 triumphs like mine, but one of them has developed a strange brake problem. just driving around locally the brakes work fine, the problem is when you get on a dual carraigeway or m-way and doing about 50mph. why 50 you ask well he's just done the big ends and mains. anyway back to the brake thing going along without any need to brake, they'll start coming on on their own. it'll get to the point where the car can't take it and he has to stop. about 10-15 mins later he can drive away. now he's changed the connector to the servo the plastic bit. the connector on the manifold, the servo pipe. even the servo has been changed. someone suggested the breather on the brake master, but that hasn't solved it. the last part to change is the brake master. when the brakes come on it's all the brakes. it's not a dual circuit. the pedal can't be pushed when the car is forced to stop. he did remove the servo pipe and put a bit of paper over it, which did ease the problem by about 85 per cent. so is there summat in the master thats causing the problem

[BikerGran]

This happens to my trike when it's been in the workshop, I always have to remind them when they've adjusted the (Reliant) rear brakes, to back them off an extra notch, otherwise when everything heats up - she stops!  But she'll go again after a 'cooling off period'.

I don't know what kind of triumph you're talking about but if it's an old un with drum brakes it could be a similar problem?  Have the brakes been adjusted lately?  10-15 mins later things may have cooled down just enough to give a bit of free play.

[renegade53]

front calipers are free, rear wheel cylinders have been replace. on the mk2 2000 range they're self adjusters on the rear. all the brakes come on. the servo is bulkhead mounted not the remote type.

[o0Mouse0o]

I've had this happen on a BMW bike and  when i was in a Trabant when it did the same thing, I also had to do 'fix' an old lockheed type master cylinder before I could use it on a project.

All of these were caused by a blocked hole in the side of the master cylinder that allows equalisation of pressure back to the reservoir when the plunger is in the fully relaxed position. Its a tiny small hole and often overlooked as a ripple in the paint or rust flake, So small infact that a bit of corrosion can block it at hydraulic pressures and you'll miss it if you'r eyes arn't good. I drilled mine out to 0.8mm and solved the problem.