at first everything seemed fine. then the brakes started to not work properly and the car would barely stop, even at low sped.
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When a booster goes out, you must increase the pressure applied by your foot as the booster is no longer boosting your muscles' power. This could make you feel like they are tougher to actuate, but they should still work, you'll just be red in the face as you try to stom down on the pedal.
Boosters are not made out of jelly or some other substance that goes bad after 3 weeks of inactivity. If that were true, think about how many people would be writing into the list or talking about it at the barber shop or whatever. There would be tests to be performed every time that you even thought about driving the car!! The rebuilder is feeding you a line, I'm afraid.
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If you are driving your 1953 with a new booster and nothing else done, it could be something else. EVERY car that I get gets new brake hoses, master and wheel cylinders installed immediately.
Before I drive it.
If one seal goes on your system, you're not going to have any brakes. You will still have steering so you'll have choices about where you land, but thats about all you'll have going for you.
If you have had everything else done, inspect all lines and wheel cylinders for leakage/failure. Open the MC and see if it's out of fluid. If it's low or empty you have a brake system problem. If full and the pedal's hard, you have a booster problem.
Could be that you have a booster rebuilder problem too.