hoping someone here can help diagnose an AC problem. with gages hooked up and about 75 outdoor temp i have 30psi suction and about 150 head pressures, but very little or no cooling. i have bubbles in sight glass, but when i try to add freon(r-12) the suction pressure won't rise at all and i don't get rid of the bubbles in the sight glass. i did notice frost build up on the small line from POA valve to evap box after 15-20 min of running. compressor sounds normal and clutch is pulled in. was thinking of replacing POA and Expansion valve, but not sure thats my problem. any ideas?