JH, what Do you do to prevent the exhaust valve from leaking? Recommendations on how long a motor should hold air pressure?

I use a inflatable bladder that goes into the exhaust port upstream of the power valve.  Checking engines with power valves is a real pain. 

Pressure checking helps find leaks that are due to cracks and leaking gaskets.   Pressure is not a good test for crank seals if you are experiencing a lean condition.  A vacuum test is what is necessary to test the lip on the crank seal that controls leaks that cause a lean condition.

I generally use pressure and vacuum test for diagnostics before tear down not after rebuilding an engine. With enough experience, you learn where to look and correct the problem areas before assembly.

An air leak that is bad enough to cause a lean condition at and idle or high-RPM closed throttle deceleration will usually need to be around a 3  to 5 psi or more per minute leak.  The TM 38 carburetors that are bored to more than 41.5 will usually have what is comparable to a leak that is much worse than a 3 to 5 psi air leak due to bad gasket or crank seal.

A small air leak will have little effect on the air fuel mixture at full throttle.  Air leaks, worn out slide bores and overbored carbs cause the most problems at partial and closed throttle operation.