Auto Air Conditioning & Heater Repair: 99 Silverdo AC stops cooling after a whi;e, hvac control, silverdo

Question
Hello David, I have a 99 Silverado. The AC will cool off the cabin but after 20-50min it will stop. I can turn off the system and re-started 20 min later and it will work for another 15 min and stops. So far I replaced the pressure switch and the orifice tube.The low pressure is around 40psi. The high pressure I don't have any idea since I was not able to find the high pressure outlet. I noticed that when it stops working and low pressure will climb up to 90-100psi even with the system turned off. After leaving it off for a while the system starts again and the pressure goes down normal. Also, I noticed the following, the AC system is not cycling the compressor. It will be blowing cold air until the system stops. When in recirculation mode if I turn the AC switch on/off it won't disengage the compressor. If the fresh air is selected and I press the AC switch it will turn on/off the compressor. I think I may have a bad HVAC control unit (suspect flood damage) or a bad compressor. What do you think? Thanks for your time.

Answer
Anthony,
First thank you for the thank you, there aren't too many that say anything back in that aspect. Now let's see if I deserve it.
First with your compressor off, your low pressure will rise back up and eventually read the same as your high side reading. This is just the refrigerant equalizing in the lines. If it didn't it would make the compressor work harder on start up pushing against that high side pressure.(Like starting a car with a manual transmission while it is in gear) So that is what it is supposed to do. No worries there.
As for why it is cutting out. If have reason to suspect the control head because of flood damage, then that would definately be a possibility. But it may be easier than that. It could be that your A/C clutch relay coil is causing a draw somehow, heating up, and disengaging it's contacts, thereby shutting down the compressor. I come by this idea because of the time it takes to shut down. If it was your low pressure switch or high pressure switch disengaging the compressor, it would kick off and then back on relatively soon. about 10 seconds for the low and about 30-45 seconds for the high. The relay on the other hand, being disengaged because of heat, will make it take longer because the relay would need time to cool down. The easiest way to check this is run the system and right after the compressor disengages, touch the A/C cluth relay with your hand. If it feels any different than the others in the box, there you go. replace it.
Other than that relay, the only other thing that I could think of is the control head itself. And that would be the next stop. Especially if you have reason to suspect it being waterlogged at one time.
I hope I was of a little help and deserve the pre-thanks. Thank you for the question and good luck.