80impala

So I'm slowly repairing my near mint 1980 impala. it has 80k on motor and trans(both original). I seem to have break pressure when the car is not running, but when it runs, I hear a loud hiss when I step on the break. We replaced the master cylinder and are thinking maybe it is the brake booster. Not entirely sure how to check if it is the break booster, any information on that would be greatly appreciated.

The car also has a nasty miss fire. I replaced sparks and wires, and am about to do the distributor cap and rotor. The miss-fire is still there, but seems less prominent now then before the sparks and wires.

The car also seems to like to glide on its own. Might need an alignment or maybe ball joints not entirely sure.

Any help would be great fellas!