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Bruce 1/24/2011

1994 GMC Jimmy SLE 6 Cyl 4.30L


I changed the master cylinder twice and still the brake pedal goes to the floor. Is the power boost bad?

I primed the master cylinder before installation. Bled all four wheels. I can apply the brakes but once I start the car the pedal goes to the floor. Is it possible the power boost is bad?

3 Answers


tnharpman 1/24/2011

Nothing to do with booster. Booster is just for power assist. When they fail, pedal becomes hard. If truck has drum brakes on rear, check adjustment. If they are to far out of adjustment or adjusters are frozen, that will give you excessive pedal travel (shoes have to move to far before contacting drums), often mistaken for hydraulic/master cylinder problem.


Herrm 7/12/2011

first bleed your mastercylinder. Then bleed the remainder of the brake hydrolic system starting with the furthest wheel from the master cylinder. That should take care of your problem One important item, make sure that the fluid reservoir is always topped up or you could put air back into the system.
Should you still have poor pedal, have the system pressure bled at your favorite shop because there have been times when manual bleeding just won't do the trick.


HouseCallAuto 7/12/2011

There is a problem with either the rear brake adjustment as tnharpman said or if you replaced calipers (not sure if you did) maybe you have the left front on the right and vice versa, A shortcut to isolate what wheel or wheels is causing the problem is using a vice grip and being extremely careful, crimp the rubber brake hoses closed that go to each front wheel and crimp the single rear brake hose where it comes into the splitter on the differential housing. That should get you a pedal if the master is good (likely). Then remove the vice grips one wheel at a time starting with the front and see when you lose the pedal and that will ID the wheel or wheels if in rear.


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