I bought my parents 2001 suburban that has been in storage. The vehicle is 99% rust free and everything works besides the A/C. So far I have confirmed that the system is full and the clutch does kick on when you turn the system on. I get warm/hot air out of the vents and can hear a hissing noise through inside the cab. Is there a common issue I should be looking for? Pressure switch, bad service ports leaking? Trying to diy this before I take it to a shop.