I have a 69 SS with factory A/C that I converted over to 134a. I updated the POA valve and replaced the hoses. I've run it for 2 Texas summers with no problems but recently, the A/C stopped cooling ( yes it's still warm in Texas!). The clutch doesn't engage so I put a gage on the low side fitting and it reads around 20psi. I added 134a to it and the pressure increased and the clutch began cycling but very quickly. I bled the low pressure side back to 20 and shorted the POA switch to the compressor and the clutch kicked in. Pressure reads close to 0 with the compressor engaged but still no colling. Does anyone have any ideas what to do next?

The stock 69 AC system doesn't have a low pressure cutout like current cars. The only thing that will interupt power to the clutch is the low temperature switch on the fender side of the suit case. The low temperature switch is the two prong electrical connector that sticks out of the side of the suitcase. It will open the electrical circuit between the control switch in the dash and the clutch when air temperature in the suitcase gets down to 29 degrees.

Do you have some kind of replacement POA valve with a low pressure switch added to it?

Operating pressure should be around 25 psi low side, 250 psi high side on an 80 degree day with the engine running around 1500 rpm. If the low presuure is higher than that, and the high pressure is lower you are overcharged. Undercharged will be the opposite.

Thanks Mark.
Yes, I updated the POA and it has a low side pressure switch on it for the 134a. I just bought a better gage set and found that the high side won't go higher than 90psi. Low side goes up to about 55psi which is too high. This is after adding 1 can of freon ( 12oz). I bled the pressure off but both sides drop. As previously stated, the system has worked great for over 2 summers.