This may be as simple as an undercharged system. When converting r12 to r134a, POE oil is added to allow r12 oil become compatible with r134a. This was the quick, down and dirty way to retrofit r12 systems to r134a but no one discusses how much r134a is needed and you cannot use the r12 sight glass as a way to determine r134a amount. With an old compressor, it's probably noisy from less than ideal amounts of refrigerant and old oil. R12 uses mineral oil and not compatible with r134a, hence adding POE oil. Ideally, an old r12 system is flushed of all old oil, the receiver/drier is replaced, new pag oil is added with dye, all O-rings replaced with r134a seals, a complete evacuation using gauges is made to determine if a leak remains or a vacuum holds after shutting off the vacuum pump. If a vacuum holds after 15 minutes, this means the repaired/rebuilt system can hold pressure. If a vacuum leaks atmospheric air back into a system, there's a leak that will not hold refrigerant. Ac repairs are not diy friendly and require refrigeration knowledge. A repaired system holding a vacuum should hold refrigerant. Adding r134 (at least one can with dye or a small vial of dye added when the system was open) is injected into the low/suction side before starting the engine. Both gauges are monitored when injecting refrigerant. One key is to add the second can a few ounces at a time then allow system pressures to stabilize. Stop adding r134a when low side pressures remains around 29-35 psi. You may have to run a high idle, around 1200 rpm, for optimum system pressures. No one drives at idle speed and operating pressures varies in proportion to engine rpm. It's easy to over fill a system with high side pressures exceeding 250 psi. Compressors have a safety relief valve to open of pressures exceed 450 psi. Low side pressure is the key to determining lowest temperature and pressure, always above freezing (32°F).
Ignore static pressures; once a hot ac system stops, high pressures continues to be released thru the thermal expansion valve that remains open. Both gauges are showing high pressures releasing thru the txv with low side pressure jumping up as soon as ac is turned off, whether from the combination low/high pressure sensor or manually shutting off ac. While the txv varies release of high pressure liquid to maintain low side pressures that correlates to above 32F (preventing creation of an ice box from moisture condensing on the evaporator coils), additional info leads me to ask you a cooling fan question. Does this system have an electric cooling fan in front of the condenser coil or behind the radiator (or both) and is/does it/do they run when ac is turned on? The best troubleshooting feedback was mentioning water spray on the condenser coils, immediately dropping high side pressures from 300 down to 160 psi; lack of cooling fan operation. Either you have an older clutch fan that's worn out and not engaging to pull in airflow thru condenser and radiator, missing/damaged cooling fan shroud or the electric fan(s) are dead/running at the wrong speed (some vehicles have two fans with three electronically controlled speeds). Ac must have fan(s) running since the compressor creates high pressures and heat at the discharge side, feeding hot refrigerant into the condenser coil in front of a hot radiator. Condenser coil and radiator must have fan cooling when ac is running otherwise overheating/higher than normal ac pressures occur and the radiator may overheat. Fan cooling seems to be the problem and you haven't mentioned amount of original refrigerant (r12) or replacement r134a, which adds to this errant ac system not operating ideally.