Pressurizing your intake manifold and engine isn't a problem at all. Factory service manuals suggest it as a way to find vacuum leaks.
Like they said above- take off your oil cap. That will relieve any pressure that leaks into the crankcase.
If you can't get the boost gauge to read any pressure the engine may be in valve overlap position, meaning both the intake and exhaust valves are open. If that happens, just crank over the engine a little and the exhaust valves will close.