: 70 DeVille convertible brake master cylinder vacuum booster always activated



douglasaorr
07-27-13, 09:04 AM
I installed a new master and vacuum booster one year ago. Recently I heard scraping noises and smelled hot metal so I checked all four wheels and sure enough the front wheel hubs were red hot. So I jacked up the car and removed the front wheels. The pads were worn down to the nib and the rotors were badly grooved and the caliper pistons were fully extended. Assuming the calipers were bad and just stuck, I proceeded with a complete front brake job. After installing new bearings and seals, new rotors and hubs, new calipers, new pads, and new flexible lines to the calipers, and after bleeding the entire system, the front brake pads were constantly dragging on the rotors and I could not spin the wheels by hand. The hubs and wheels became very hot just driving around the block on a test drive. Only after disconnecting the vacuum supply hose from the booster could I spin the wheels and tires by hand and after driving it without power assist did it relesse the caliper pressure on the pads and rotors and do the wheels spin freely and not get hot. It became apparent that the booster was constantly slightly activated. I've read in other GM car forums that it's possible the push rod from the brake pedal to the booster may be too long. But I can't find any adjustment for it. Does anyone know the solution?

Neil Clarke
08-01-13, 07:08 AM
If its recently occurred but wasn't an issue before (but after you fitted the replacement units) I'd say the booster could be holding the brakes on due to contamination or a fault in the booster
If the one way check valve is not operating correctly every time you turn the engine off the booster can suck petrol fumes back from the manifold
This eventually causes the booster diaphragm and seals to swell holding the brakes on

Might need another booster and one way valve