a/c compressor running low

I have a single phase copeland compressor that runs fine for about a half an hour then the high side goes to 275 and the low side starts to go down to 15 psi. it's like try to run on vacuum but then it shuts off of course. Condenser is clean and it has capillary tube.
when the pressure is running right it stay around 250 high side and 35 degree coil. I tried to get it to 40 degree coil but the head pressure goes to high. Do you I have too much on it? the temperature outside is 92 and is a r-22 unit. any help would be appreciated:mad:

If your suction pressure drops and your head pressure goes up at the same time it sounds like you have a restricion in the system somewhere that the liquid is backing up behind, such as a plugged drier or cap tube, the fact that the head pressure rises also makes me suspect the system has been overcharged to try and bring the evap coil up to propper temperature.
I'd be changing the drier and if it seemed ok checking the cap tube or piston. You may have to reduce the charge after finding the problem, i always mark the unit with the aditional charge over factory spec, makes it easy for the next guy.
Check your evap coil and filters too and air flow.

Jon

could there be moisture in the system? if the unit runs ok for half an hr then develops this problem is moisture causing the capillary to block up with ice

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Originally Posted by razor007

could there be moisture in the system? if the unit runs ok for half an hr then develops this problem is moisture causing the capillary to block up with ice

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Yes, it could be! Check that by heating capillary with hot air gun. If suction pressure rise and discharge drop, than it is sign of moisture in capillary which is freezing.

the first thing I would do is recover the charge, carry out a triple evacuation and recommision with new drier.