hi, actually the evaporator core reach the 100psi when the system is turned off. the pressure stabilize on the entire system once is off. an evaporator core can be tested with the valve on and I recommend that you test the evaporator with around 150psi. Sometimes when the leak is minimum the low pressure wont let you detect it. when I test an evaporator, I turn on the system for around 5 minutes, then I turn it off and wait around 4 minutes, after that I turn on the blower only, not the compressor, and check for leaks with a leak detector.