A/C compressor preassure valve keeps engadging

The A/C is either overcharged (too much refrigerant) or there is insufficient airflow across the A/C condenser up front of the radiator. Either one will cause an overpressure condition that will make refrigerant vent from the pressure relief valve.

Proper refrigerant charge with R12 (original FREON refrigerant) is 32-34 oz.
Proper refrigerant charge with R134 or Freeze 12 is 26-28 oz.
The above figures do not incluse the 6-8 oz oil charge needed to lubricate the A/C compressor.
Many of the aftermarket refrigerant recharge kits will overcharge the A/C if used without a set of charging gauges. This is particularly true if you top off a system that is low on refrigerant in an attempt to bring the A/C system performance up to par.

If the refrigerant charge is OK, then your fan controller isn't set up to run the fan anytime the A/C is on. The fan needs to be running, even if it is at low speed when the A/C is turned on.

If your refrigerant charge is up to spec, then try placing a large floor or box fan running on high speed in front of the car with the engine running and the A/C on. If it doesn't vent refrigerant, then you have a fan controller problem.

The fix is to purchase a high end fan controller like a DCControl (www.dccontrol.com) or mount a small fan directly on the A/C condenser that runs anytime the A/C compressor is running. Isuzu Rodeo's had such a fan that was wired in with the A/C compressor clutch