brakes dragging

Hello, I have 1955 f100 pickup volare front end lincoln rear both disc brakes 8inch power booster gm 1inch dual master cylinder brakes dragging when driving down road come back to shop crack brake line unlocks wheel pressure building some how it is new master cyl which dosen't mean much now days I am tryng to find which stock gm has 1inch cyl with 3/16 ports so i can buy cheap at parts store to try and eliminate m/c I have taken all external valve out of system trying to solve problem there are no collasped hoses Thanks MIchael

Welcome to CHR. You have done a great job describing your problem. Iam sure someone here can help you. Hope you enjoy CHR.

just to start, are you sure you have a disc/disc master cylinder? are you sure you have a power brake master cylinder (not a manual brake cylinder)? your symptoms are common to using the wrong master cylinder.

Here is a couple of hints: Master Power Brakes - Master Cylinders

mike in tucson

Your booster could be pre-loaded. Is your pedal linkage adjusted to the correct length so that you have a bit of clearance at the booster diaphragm, and a spring return to keep the pedal weight from pre-loading your booster?

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Originally Posted by robot

just to start, are you sure you have a disc/disc master cylinder? are you sure you have a power brake master cylinder (not a manual brake cylinder)? your symptoms are common to using the wrong master cylinder.

Here is a couple of hints: Master Power Brakes - Master Cylinders

mike in tucson

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yes this is a disc.disc m/c and the booster has clearance to m/c pedal has clearance, booster unhooked from vacume, all wheels are free go down road 2 miles start gaining pedal brakes dragging ??? come back cooled off same thing over again