I feel kind of embarrassed that I have to ask this. I am having a problem with a 67 Cougar and its A/C. A friend of mine installed an new R-134a system into his vehicle and asked me to charge the system for him. I first put the system under a vacuum and it held. I then slowly charged the system with vapor to 30 psi on the suction side. The system ran fine for about 20 minutes, then the pressure jumped up and pegged out the high and low side gauges on my manifold and the relief valve lifted on the compressor and dumped the entire charge. I then put the system back under a vacuum. Again it held. I then put a can of oil charge in the system, then charged the system with vapor refrigerant to 25 psi. The system ran good for about an hour or so, then did the same thing again. The car does not have the economizer, but everything else is good to go and the system is sound. Does anyone have a suggestion on something I might have overlooked, or have missed, or can do to get the system working. Thanks in advance.