MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - brake pedal suddenly stops working

hi guys, my 68 gt has developed a strange feel in the brake pedal. When i press down and hold the pedal i can feel it pushing back on my foot, sometimes i cant hold it down and when it reaches the top, thats it, no brakes as i cant push the pedal. I have to literally lift off the brake altogether and then press it again. Ive also noticed that the brakes operate at different levels of pedal travel. one moment i can press it just an inch or so, other times my foot is near the floor. In 2 years i havent had to top up the fluid, its never leaked. Any ideas? i was thinking maybe the servo or the one way valve?
A P New

I've never heard of the brakes forcing the pedal *up* while pressing down before!

Different levels of travel could be the master seals about to give way altogether, but can also be the servo sometimes working and sometimes not. But the pedal going down only an inch sometimes, and nearly to the floor on others, sounds a lot more than the usual difference a servo makes.

An intermittent servo *could* be the one-way valve on the inlet manifold that the servo vacuum hose is connected to, but unlikely I'd have thought.

As the servo only gives light assistance (it was optional) probably the easiest thing is to remove the vacuum hose from the servo and block it e.g. with the pointy end of an old spark plug, and see what effects you get then. But carefully, bearing in mind it could be the single-circuit master!

PaulH Solihull

ill give that a try tonight on the way home. ill take it slow of course lol.
A P New

ok i took the servo hose off and took it for a spin. the pedal was incredibly stiff (im guessing stiffer than a normal non servo assisted car as the fluid has to travel through the servo also). Icouldnt get the pedal asfar down as with the servo and the pedal did not try to push*tself back up. Im guessing the servo may be at fault (it was second hand 2 years ago when i got it). Any thoughts from anyone else?
A P New

ok last night i took it for another drive, i managed to lock up the rear wheels with no servo. i then stopped the engine, put the hose back on the servo and slowly sucked on the hose. i then blew back into it and found i could no. i assume thats the one way valve working. I put everything back together and took it for a drive. the brakes were suddenly as they should be, no strange pedal travel, no fighting back. weird.
A P New

The stiffness - or more correctly harder pedal needing more pressure to get a 'normal' braking effect - is simply from there being no servo assistance. With consistent operation like that it would tend to confirm the servo is at fault.

However the one-way valve is inside the inlet manifold port. This should mean that with the hose removed from the servo put still on the manifold, you should be able to blow in the hose but not suck.

If the hose is on the servo you shouldn't be able to get any real air flow by blowing or sucking, unless the servo is faulty. Only by sucking on the hose with the brake pedal operated might you move the diaphragm, but you would need a helluva pair of lungs!
PaulH Solihull

I've experienced this long ago, don't recall what sort of car. Very strange and will result in disaster sooner or later. I think it has to do with an internal seal fault in the servo cylinder, which can only get worse. The boosted side leaks past the seal to the unboosted side, while the servo continues to operate because the MC is maintaining pressure on the servo valve. The boosted side has a larger area/volume than the unboosted side, which is what causes the pedal to rise. When the servo cylinder gets to the end of travel, everything quits working. At that point the system will have whatever pressure was on it before, which may not be what you need, but you can't change it without releasing your foot and letting everything return to rest position. That may take more time/distance than you have. I will note that the phenomenon is more likely under light pedal pressure than heavy, which reinforces the seal leak idea.
I think you might find that this (light pressure/rising pedal)is in fact a common test for servo malfunction, but I have limited experience with servos so haven't kept up. It is very disconcerting and certainly means that things in the brake system are not well.

Either rebuild the servo or remove it entirely. In 68 the system was identical w/wo servo, other than the servo itself, so it will work like any other early B; you'll get used to the increased pedal effort shortly.

FRM
Fletcher R Millmore

This thread was discussed between 26/07/2010 and 28/07/2010

MG MGB Technical index

This thread is from the archives. Join the live MG MGB Technical BBS now