AC blows all the freon out
 

I vacuumed my system down for 4 hours. Then left it to see if it would hold a vacuum. Ten hours later still holding a vacuum. So I started adding freon. Seem like it was taking forever but stayed with it. Got the system up to 28 lbs on the low side, couldnt get a reading on the high side (yes this car been converted to 134a). The high side I believe has something to do with my connector not piercing the fitting.
Anyway, I started adding some more 134a my chart showed I should have 35-40 on low side. During the last can and low side around 32 all of a sudden something lets loose and all the freon blows out under the car. I tried to look where it was coming out of but it was such a fog I have no idea. I jacked the car up and crawled under car and have no idea what let the freon out. I checked fittings, hoses. I did find a bad oring and i replaced it.
So I pumped it down again and started filling it, and yelp it did it again. About same pressure. Fittings on back of the compressor seemed dry.
Car is a 1990 Volvo 240. Compressor looks varily new. Hoses look good. Does my car have a high pressure relief valve? Any help will be appreciated.

I don't know on the 240's but on the 740 under the front passenger side fender there is a high pressure relief valve in the AC line. It is in front of the wheel down below the the headlights under the car. It is to prevent over pressurization of the system. The funny thing is when I replaced mine because it would blow off once in a while the new hose assembly didn't have the valve in it. It is part of the metal/rubber line that goes into the condenser. If it blew it should be wet as some of the system AC oil will come out.

The pressure relief valve will be somewhere on the high side of the compressor (it may be on the back of the compressor or on a high side line somewhere)

it'll below at something about one 450psi I believe

if you over-charge the system, it'll blow
if you compressor isn't flowing properly, it can also over pressurize the high-side and blow

depending on your compressor, you will have a high pressure service port on the back (top center location) with a hex bolt or a metal cap I believe this is also the pressure relief valve, but again, it all depends on what compressor you have

update
 

Well did a little more testing today. I think I figured it out. As the ac runs the low side keeps going up but the high side stays really low (50lbs.). I beleive that either my condensor is plugged up or the expansion valve plugged. Im going to keep ac off and do a major rebuild. I believe compressor probably shot. So what is a good brand of compressor? I have a new condensor on it way. Thanks for advice and help.