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MG MGB Technical - brake help please

after inspecting my brake system on my 76 bgt post brake failure. no obvious sign of a leak anywheer and the fluid level was'nt really that low(probably 3/4 full)so pumped the pedal a little and pressure returned. then tried to start the car and sounded and felt very rough(not normally like this) and smoke came from the exhaust(again not normal).
reved the car 2-3 time smoke stoped started to run smoothly again, but no pressure in the brake pedal. push pedal back to rough running.
any ideas on the problem/cause of this.

jr williamson


seems that there is a leak at the end of the master cylinder so that brake fluid is leaking into the brake servo. When the engine is running, it is sucked from the servo housing into the inlet and produces the rough running and the smoke.

A rebuilt of this type of MC is easy and straight forward.


Does sound like the servo seals/diaphragm failed. UK didn't get the integral servo until the 77 model year in September 76 so this could be either type of servo. In fact I didn't think the integral type could suffer this type of failure, as the servo acts on the pedal push-rod and not the fluid like the remote servo does. However this site does put forward the suggestion that if the primary master seal were leaking, *and* the servo output seal is leaking the servo vacuum will suck fluid from the master into the servo, and from there it can get into the engine.

The thing that would concern me though is that I've never heard of this causing total loss of brakes i.e. the pedal going to the floor until the master reservoir has been emptied. However if it is the split-system master then there is an internal baffle between the front and rear sections so that a leak on one section can't completely empty the master, but only its section. Maybe the fluid is just up to the top of the baffle on the front only and that is what you can see.
Paul Hunt

PS. If it is the remote type then you could bypass the servo altogether for a while, to see if you have a problem with the master as well as the servo. In my experience having driven V8s both with the remote servo and with it disabled it only adds very light assistance (it was only an optional extra for a time after all) and can easily be driven without. The integral servo is a different kettle of fish, though, adding much more assistance and hence being very heavy without.
Paul Hunt

thanks for the help and advice. turned out to be a small master cylinder leak into the remote servo which nackered the seals.
jr williamson

"master cylinder leak into the remote servo"

Eh? There is only a fluid link between the two. How could a leak in the master cylinder affect the remote servo?
Paul Hunt

This thread was discussed between 03/11/2008 and 07/11/2008

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