Mine does that every once in a while on VERY hot days when idling. It is because the G body system can not keep up with the heat exchange rate of the R134a and the pressure pops the relief valve. I solved this problem by not overfilling my system AND adding a couple fans to the condenser.
Are you using r12 or r134a?
The main problem with the r134a is that it has to be compressed higher to be as functional as the r12. The r134a is made of smaller molecules. Because of the higher pressure the system is more prone to exceeding the manufacturer's tolerances that they built it to handle. You need to do some work to get the system "better" than what r12 likes. And even then it still won't cool as efficiently as r12 used to.
1. Make sure you are using the right amount of r134a. I forget the conversion ratio, but you are not supposed to use the same amount of it as you would have used r12. Note that some of those CANS they Autozone sells has a little gauge attached that has a "green" zone where it should be "safe". While that zone might be "Safe" for a newer car that was designed to use r134a, it might actually be at a pressure that your 25-30 year old car can not safely function in. If the guys are going off the OLD r12 pressures and filling it with the same amount of r134a then that would be overfilling and making the valve pop.
2. Make sure you have all GREEN seals on all your hoses. Those are for r134a.
3. All your hoses have to be dual layer hoses, you can not just reuse the old ones. (You CAN, but you will be refilling your system more often. The smaller molecules leak out the non-dual layer lines more easily)
4. Someone should have vacuumed out your lines, including the compressor, condenser and evaporator, used a solvent, used a high pressure hose and evacuated the lines, then re-vaccumed the crap out of them and put in a NEW desiccant can before filling it with R134a AND the appropriate amount of PAG oil.
5. Lastly, replace or adjust the pressure sensor attached to the desiccant can. The one that is on there was made to control r12. There is a small adjustment screw on the valve sensor. You need that dual gauge manifold set to adjust it. It should open and close at different pressures than it did with r12. I forget what they are off the top of my head. If the sensor is malfunctioning and letting the pressure get very very high then that could also be why the system is popping.
Did you or someone else use any kind of "sealer"? That might have clogged something down line from the compressor and it is backing up. If you compressor is going bad that might also have send shards of metal into the lines and clogged your expansion valve (orafice tube)
