Expert: Brad Sears - 4/1/2008

Question
QUESTION: I HAVE A 1955 CHEV. I RESTORED I PUT FRONT DISC BRAKE CONVERSHION KIT ON THE FRONT JUST CALIPER AND ROTORS USED ORIGNAL SPINDLES THE KIT WAS FOR ORIGNAL SPINDLES TO BE USED I DID NOT CHANGE THE MASTER CYLINDER AND THE DISC GOT REAL HOT YOU COULD SMELL THEM I FOUND OUT I HAD TO PUT A UPTODATE MASTER CYLINDER AND PORPORTION VALVE ON IT I ALLREADY HAD THE BOOSTER ON IT THE BRAKES WOULD STOP ON A DIME WITH THE OLD SINGLE MASTER CYLINDER ON IT BUT AFTER INSTALLING THE DOUBLE MASTER CYLINDER AND PORPORTION VALVE ON IT THE BRAKES ARE NOT MUCH GOOD AT ALL I HAVE TO PUSH REAL HARD ON THE PEDAL TO GET IT TO SLOW DOWN OAND STOP I HAVE BLEED THEM A DOZEN TIMES AND IT DOES NOT HELP COULD IT BE THE ROTORS GOT TO HOT BRAKE PEDAL IS HARD BOOSTER IS WORKING ALL PARTS ARE NEW CAN YOU GIVE ME SOME ADVICE ON WHAT TO DO?THANKS

ANSWER: There is more differences between the master cylinder for disc and drum brakes beside the proportioning valve. The drum brakes use a smaller bore in the master cylinder since there is more braking active area on drum brakes they can use less hydraulic force. The piston in disc brakes is very much larger than the wheel cylinder in a drum brake and requires more volume of fluid per stroke. Poser brakes are recommended for all disc brake applications. Since you had a drum brake booster it is very possible that you will need to revalve this booster to provide more pressure. Have you checked the action of the booster after changing the master?
Brad

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QUESTION: i replaced the booster when i installed the porportion valve it is supose to be the right booster but mabe its not WHAT DO YOU THINK?THANKS

ANSWER: It could be the wrong booster or a defective booster. Now, check the booster. With the engine off pump the brakes until all the stored vacuum in the booster is gone. Now hold down the brake pedal and start the engine. The pedal should fall out from under your foot a bit as the booster loads up. If it does not check the vacuum hoses and one way check valve.
Brad

---------- FOLLOW-UP ----------

QUESTION: i pumped the brakes with the engine off then started it had foot on the brake pedal eased down about 1 inch did not drop fast what do you think/THANKS

Answer
That's fine, the booster looks good. It is possible that the front pads have been destroyed by the heat. I don't think that you have hurt the rotors as they will go out of round and cause a pulsating brake pedal. Now it is time to check with the company that sold you the parts and present your problem.
Brad