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MG MGB Technical - 76 B Brake Problem

hey guys my 76 B is still haveing problems. I rebuilt the master cylinder but im not familiar with the power boster since my other B it a 68. When i pump the brakes 3/4 of the way i can feel its giving very minute pressure not enough to tighten the brakes. but if i push the brake peddle to the floor after i pass the 3/4 mark it makes an air poping noise and all pressure disapears and it goes to the floor. however when i have the car started theres no pressure at all but it doesnt make that poping noise. iv tried everything i can think of any ideas would be great
Scot Hamm

Are you losing fluid when you are hearing the popping noise?
Did you replace the flex lines?
Are the rear brakes properly adjusted?
Generally when the booster fails you get a hard pedal.
How did you bleed the hydraulic system?
Pedal going to the floor is indicative of a bad master cylinder, poorly adjusted rear brakes, a leak in the system, or air in the system.

Have you removed the front calipers? They can be refitted on the opposite sides preventing proper bleeding. Sounds simple but can take a long time to find!
Neil McGurk

Just read your description again. Stupid question, but could the seals in the master cylinder have been fitted opposite way round? Just realised how that looks, I'm not fixated by opposites really!
Neil McGurk

And 3/4 the way to what? To the floor? Or to the point at which you can feel resistance? The pedal shouldn't get anywhere near 1/2 way to the floor. If it is going to the floor either the hydraulics are full of air or there is a leak. If the leak is anwhere other than the master seals there should be visible fluid loss. If there is none then I'm with Neil's opinion of incorrect or incorrectly fitted master seals.

Operating the brake pedal with a stopped engine and a *remote* servo makes various hissing and clicking sounds until the vacuum in the reservoir has been dissipated, can't speak for integral servos on an MGB.
Paul Hunt 2

I am having a similar problem with my brakes - unexplained loss of fluid with no obvious leaks, soft pedal which responds to pumping. I replaced my master cylinder some months ago with a new pre-assembled unit and the system was fully bled and has been working fine up to now. I am told that I have a booster problem - what is it and can I fix it myself ?
Tony Bruton

i ordered a new master cylinder in today A1 Cardone parts if anyone ever needs one is a lot cheaper only 80 bucks instead of the 260 that moss wants hopefully it works after this

Tony - loss of fluid with no visible leaks usually means that the seals in the booster have gone and the fluid is being sucked into the engine. If left unchecked this can cause total brake failure. However this is on the system with the remote servo, not any of the integral servos as far as I know. Being in SA and not giving the year we don't know what type you have. As a temporary measure you can disconnect the vacuum hose from the inlet manifold (and seal the inlet manifold port of course), in my experience the car is perfectly safe without the *remote* servo, it was optional anyway.

Soft pedal that responds to pumping usually indicates air in the system, or slack rear brakes. The former can be caused by fluid loss, of course, even when topped up, if the level in the reservoir gets down to the pressure cylinder.
Paul Hunt 2

This thread was discussed between 31/12/2006 and 05/01/2007

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