1994 Other Toyota Models aircon stops working after about an

Air Conditioning problem
1994 Other Toyota Models 4 cyl Two Wheel Drive Automatic 130km miles

i have a 94 toy hiace super custom van import that has dual air con but 1 compressor and has been faultless for 2yrs till now, the front air con is mostly working on cold days for about an hour then it dies off, yet if I turn the back one on its always 100% fine. To get the front working again I need to park up for a couple of hours. If I leave van out in the hot sun and then turn aircon on the front one wont work but back one is still fine.
i have found evaporator and washed it and the drain is not blocked and I dont think it ices up. I have found that there is water draining when working but stops leaking when in faulty state, rear drainage always works, if front air con only is used and is in faulty state I believe the compressor stays on, also I dont see anything in the little gas glass window next to the drier
hope this is enough info and not to confusing
many thanks
jay