AirLift won't hold vacuum on drained system.

[antny]

So I am about to do my water pump and thermostat because the car is running a little hot. The car is fine if I run the ac and the fans kick on and the temp does not climb. If I do not have the ac on and the fans are not running it will climb slowly. My coolant level is fine so I ordered a uview and hooked it up tonight to check the system on a vacuum. So my question is what size compressor are you guys using? I borrowed my in laws compressor and it is a 3 gallon and is pressurized at 100 PSI to start. Once the air starts flowing it will get to 25 psi of vacuum but at that point the air compressor kicks on and jumps back down to 20-18 PSI and will flicker. My system does have fluid in it still. Do I need a bigger compressor that will hold the PSI more or do I close the valve as soon as it hits 25 psi and wait to see if it holds the vacuum?

Thanks!