Auto Air Conditioning & Heater Repair: A/C relief valve, pressure relief valve, compressor clutch

Question
I have a tandem dump truck and was trying to recharge the a/c and at first the compressor clutch would kick on for about a second so I added some freon using a indicator light gauge. The gauge kept reading low so I kept adding and the clutch kicked in more often and for longer but the low light was still flashing so I put more in. I kept doing this until the clutch kicked on and stayed on but light still read low so I added till it read proper charge. Now when I turn ac on sometimes it will blow off out the back of the compressor, is this a normal occurance due to an overcharge situation?Bad compressor or fitting?

Answer
The only way to surely know if you have a bad pressure relief valve is to check it when it blows off with high side pressure gauges.
 In 20 years of doing this I have rarely seen the pressure relief valve to be mis calibrated.  I see them leak all the time but I don't recall one ever blowing off at the wrong pressure..

I think you have over charged the system.  Recover refrigerant until system starts to cycle.  That way you know your low side is getting low enough to turn compressor off.  Then you can add small amounts until system performs adequately.