The cylinder is not perfectly sealed an there will be leakage. You need enough compressor flow/stored air to maintain some pressure, but you don't need 100 psi, and 20 psi may be enough.

If the engine is at TDC and a valve drops you should be able to pick it back up by the valve stem. Mark the balancer in 120 degree increments, then starting with #1 TDC, move the crankshaft in 120 degree intervals and change the seals in the firing order.

Prior to compressing the valve, place about a 3/4" socket on the valve retainer and give is a tap with a hammer. This will break the retainer loose from the locks and allow the spring to be compressed without taking the valve with it.

Duke