I would hate to embarrass myself by showing a picture of what I cobbled together for my pressure tester, and it works with a bicycle pump..

Think short 3 inch piece of black pipe, with Schroeder valve threaded into one end and a piece of hollow brass plug with a steep for an o-ring and threaded 3/8-16 threads to go into the gear case.
Then bought a 15 psi MAX air pressure gage and threaded it in the middle of the pipe..