Make up some soapy water, apply the pressure and paint the soapy water over all the gaskets, seals and joints you can find, first check your test device connections, the plugs on the exhaust, the plates on the reedcage, the hose, the pressure gauge, everthing. The bubbles are normally easy to see and hard to miss.
It should hold 6 psi about 1/2bar for 6 minutes, do not pressurise higher than this.
Any loss of pressure of more than 1psi in 1 minute is a problem.
If the test equipment is air tight, check the engine, base gasket, case joint, etc
Next, check the right hand crankseal for leaks at the fitting for the gearcase breather, no air should leak out that hose.
Finally, if you are sure there is a leak, but you haven't found it, remove the flywheel cover and the flywheel and check the crank seal behind the flywheel also.
Later
J.J.