my 1970 chevelle when i am at a high idile crusing down the road the brakes are very touchy, but when i get to a stop sign it doesnt feel like it will stop. It has a booster on it with a new master cylinder. I bleed all the brakes so there is no air in the lines. What could be my problem? thanku

Maybe the master cylinder is a rebuilt that has been bored out. Larger bore means less mechanical advantage ergo harder pedal to stop the car at slow speeds. Mine does the same thing especially when going downhill at maybe 15 or 20 MPH.

My question is where can I buy a master cylinder with the correct bore size??

Any chance the check valve or the booster is leaking? Lot's of vacuum while cruising, drops when slowing down. The booster should hold the vac even after shutting engine off if there are no leaks.