Why are my brakes so sensitive?
I recently did a rather complete restoration of the engine compartment of my 1969 Buick GS convertible. I had the engine rebuilt and installed a new master cylinder, brake lines, etc. The brakes are now super sensitive and will throw you into the windshield if you aren’t careful with the motor running. If the engine is off the braking ability is marginal, similar to any other auto with power brakes. I used the same proportioning valve in the hookup with new brake cylinders in the front (the car has front drum brakes, not disc) and rebuilt the rear cylinders. Is there something in the vacuum boost that could be awry or would a master cylinder from a disc brake auto make this change in the braking characteristics of my car?
Hypersensitive brakes in a car with power assist is usually due to incorrect geometry in the pedal-to-booster linkage. There are normally two different attachment points for the rod that connects to the pedal, with the lower point being for power brakes and the upper hole being for manual. If the rod in your car was mistakenly installed in the upper hole, the brakes will be extremely sensitive.
An incorrect or malfunctioning master cylinder can also cause the over-sensitive condition you are experiencing. A larger power booster will deliver increased assist so, in theory, a booster that is way too big could cause the brakes to be too aggressive but I doubt that this is your problem.