Air condition 95 SL1

Thats pretty high if the system is not running. Average pressure on the low and high side should be near outside air temp.
Normal low side is 30-40 psi and high side is 200-250 psi when the system is running and the freon charge is full.

For charging purposes, you really should be monitoring the low side pressure. Depending on ambient air temperature, the low side should be between 30 and 50 lbs PSI. The high side can be between 135 and 330 lbs PSI.

Ambient Air Temperature
Relative Humidity
Service Port Pressure
Maximum Left Center Discharge Air Temperature
Low Side
High Side
13-16°C (55-65°F)
0-100%
150-246 kPa (22-36 psi)
730-1190 kPa (106-173 psi)
12°C (54°F)
19-24°C (66-75°F)
Below 40%
150-280 kPa (22-41 psi)
900-1390 kPa (131-202 psi)
14°C (57°F)
Above 40%
179-137 kPa (26-46 psi)
990-1500 kPa (144-218 psi)
16°C (61°F)
25-29°C (76-85°F)
Below 35%
218-328 kPa (32-48 psi)
1240-1640 kPa (180-238 psi)
17°C (63°F)
35-60%
236-342 kPa (34-50 psi)
1290-1680 kPa (187-244 psi)
18°C (64°F)
Above 60%
250-365 kPa (36-53 psi)
1340-1760 kPa (194-255 psi)
20°C (68°F)
30-35°C (86-95°F)
Below 30%
266-386 kPa (39-56 psi)
1500-1940 kPa (218-282 psi)
21°C (70°F)
30-50%
280-397 kPa (41-58 psi)
1540-1990 kPa (224-289 psi)
22°C (72°F)
Above 50%
300-430 kPa (44-62 psi)
1600-2070 kPa (232-300 psi)
24°C (75°F)
36-41°C (96-105°F)
Below 20%
328-439 kPa (48-64 psi)
1810-2240 kPa (263-325 psi)
25°C (77°F)
20-40%
337-457 kPa (49-66 psi)
1840-2290 kPa (267-332 psi)
26°C (79°F)
Above 40%
355-474 kPa (52-69 psi)
1890-2340 kPa (274-340 psi)
27°C (81°F)
42-46°C (106-115°F)
Below 20%
391-491 kPa (57-71 psi)
2130-2500 kPa (309-363 psi)
29°C (84°F)
Above 20%
397-508 kPa (58-74 psi)
2160-2550 kPa (313-370 psi)
30°C (86°F)
47-49°C (116-120°F)
Below 30%
451-552 kPa (65-80 psi)
2420-2790 kPa (351-405 psi)
33°C (92°F)

low side should be about 30 psi at idle speed, high side 120 to 180 psi depending on ambient temp. higher temp, = higher pressure, as you increase engine rpm to say 1000 low side should drop to 10 to 15 psi & high side should increase about 30 psi. If high side goes over 280- 300psi you have contamination (air) in system or over charged with refrigerant. you will need to recover freon, evacuate & recharge system with proper amount of refrigerant. If the low side drops below 0psi or into vacuum then the refrigerant level is low, you will have to check for leaks , repair leaks & refill refrigerant.
Hope this helps
Paul T

If there is a restriction in the a/c system, the pressure may be getting either too low or too high for system to work properly.

Also, if the charge is low, the compressor may not turn on 20 minutes into your trip.

Check to see if the pressure on the low side is 25 psi to 45 psi when running the a/c.

Good luck.

Add more freon, should have a low side reading at about 30-35psi at about 1500 rpm and stay there, if it drops low pressure switch will cut a/c compr. off.

Recharge the system through the low side fitting, with the engine running you will need to have 25-30 psi on the low side gage, add freon a little at a time until this pressure is reached with the engine running and the compressor engaged.

The a/c requires r134a freon the low side fitting is on the a/c accumulator on the passenger sideof the engine compartment. Charge through the low side until the comp energizes and the low side pressure is around 25-30 psi ( with the comp on )

You may have created an over charged condition and at this point the a/c compressor will cut off on high head pressures. This can be verified by installing gauges to the systems high and low side service taps and reading the pressures.An overcharged system will start the compressor briefly but soon cuts off once pressure goes to 350 - 400 lbs on the high side. Acceptable pressures for this system should be no greater than 250 high and low suction should drop to 40-50 psi.You might recover the charge and evacuate the system,then try bringing the charge back to proper level.A sight glass in the reciever drier really helps determining proper fill.Once all bubbles have dissipated from the sight glass during charging ,one should stop at that point to avoid overfilling.Too much freon is worse than a low charge especially in hot ambient conditions....

28 TO 30 PSI ON LOW SIDE. HIGH SIDE PRESSURE 190 TO 200 PSI.