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Braking Troubles

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79MGB16

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The brakes on my B are acting up. It seems that when i down shift and the engine rpms go up, the brake pedal gets soft and goes all the way to the floor. Anybody know what would cause this?
Thanks
 
A vacuum leak in the booster? This is purely a guess, but there might be a small leak, the higher RPMS cause more vacuum and make the leak worse. But I can't imagine it would make the pedal go to the floor. I would get that checked out asap. Brake failure can be disaterous!
 
Booster trouble can not make a pedal soft. If the booster fails to boost, the pedal gets firm. If the pedal gets intermittantly soft it could be that the brakes are overheating, although they usually don't recover from this without bleeding. Another possibility is something causing the pads to be returned too far (loose bearings allowing the rotors to wobble). If you never have a soft pedal with the car stationary then the problem is not air in the system.
Any help?
Tom
 
I would suggest it may be a combination of problems. The hydraulics are probably the main problem. the master cylinder may be starting to fail or there may be a leak elsewhere in the system that would let the pedal go to the floor if pressed hard enough.This needs to be adressed first. But the hard enough may only happen on decel when the vacuume supply to the booster is the greatest. Normally a booster stores vacuume so low engine vacuume is never noticed,and you get full assist every time, but if you have a small leak then vhe vacuume could bleed down and you may not have the full assist unless you have the extra vacuume of decel.On a modern car a bad booster creates a very "hard" pedal but on an MG the difference was less,( you get a firm pedal but not "hard") especially if you're not really losing all of the assist.
This is just a guess, but I would inspect both components. Your corrilation of this only happening under decel is Why I'm trying to link the engine (vacuume) to the brakes. The only connecting point is the booster.Even though as stated above this almost always causes the opposite condition.
P.S. if the booster is leaking vacuume, then pinching off the vacuume hose to it should noticably change the way the engine is running.
 
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