brake booster problem

I read several topics in this forum but none really defined a problem I am seeing with my brake system. I am running a 7" single diaphram, 1" bore Corvette style M/C which is under the floor. With the motor off, the pedal feels good, not spungy or low. When I start the car and step on the pedal it drops several inches and becomes rock hard. The pedal always retracts but I have nothing to about 75% pedal throw then its hard. All the components in the system are new but that doesn't say much. I am pulling about 15 psi vaccum at idle and I am thinking the something in the booster might be bad, the check valve is bad or a return spring would help.

Does this sound like a bad booster problem?

hi. I ran into this before. first thing is do you have drum all the way around? do you have frt disc and rear drum? or do you have disc all the way around? first thing that you have to check is rear brk adjustment, it must be adusted right. put a very slight drag on it ifyou have drums. if you have disc's then the free wheel gap must be very slight to the rotor when right.same for the front. the rear may or may not have the e-brake built into the caliper, if it is in the caliper the ratching internal parts have to work properly or the gap will not adjust the space between the rotor and the pad surface which will give you the problem that you are having. next is the lenght of the rod that is between the m\c and the booster, must not have any pressure on thr m\c or things get bad in a hurry.hope that this helps you alittle. good luck dave

Do you have brakes when the pedal goes hard, or is it bottoming?

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Originally Posted by mrrilla

I read several topics in this forum but none really defined a problem I am seeing with my brake system. I am running a 7" single diaphram, 1" bore Corvette style M/C which is under the floor. With the motor off, the pedal feels good, not spungy or low. When I start the car and step on the pedal it drops several inches and becomes rock hard. The pedal always retracts but I have nothing to about 75% pedal throw then its hard. All the components in the system are new but that doesn't say much. I am pulling about 15 psi vaccum at idle and I am thinking the something in the booster might be bad, the check valve is bad or a return spring would help.

Does this sound like a bad booster problem?

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I had the same problem with a dual diaphram booster. The pedal ratio was 6:1 which is for manual brakes. The ratio should be 4:1 if you are going to run the booster. I changed the pedal ratio and problem was solved.