I did just 15 psi yesterday morning with the loaner kit from Advance Auto Parts and still had a gusher at the plastic thermostat housing on the infamous coolant outlet duct assembly. That's okay, though. The XK8 is parked, I have the new coolant outlet duct assembly in hand, and I hope I can pull this nasty job off tomorrow....

 

I can set my compressor down to an accurate 10psi and adjust it up from there. I used it at 12/14 psi. No KABOOM. I have used it on my 2 Jags with great results. I've been doing most of my own mechanical work for about 50 years now. Just getting too old to do some of it now....ratz.
Will

 

The issue using a pressure tester on these cars always comes down to the connector to the reservoir tank. Neither Advance AP or O'Rilley's rental equipment has a connector specifically designed for this car. The universal, expand mechanically connector works just fine.

 

The kit I bought at Harbor Freight has the correct fitting.