351 efi E150 van - A.C. broken

I have a 1990 e150 w/a 351 efi engine. I use it for hauling my family around, but the air conditioning is broken (very hot in summertime). . .gets around 100 degrees + humidity here in Louisiana in summer. The AC is vital in this part of the world, so here's the problem:

First I thought it just needed freon, so a friend told me to buy the r134 conversion kit as the a/c had no pressure at all in it. I charged the system, but the compressor made a loud noise (like an airplane taking off). I put the entire can of oil in first as per the directions, and then put in the freon. The system cooled the vehicle, but the rear a/c worked better than the front (much colder). I didn't use it again b/c of the loud noise in the compressor for about a month. I just checked the compressor again, and the system now has no pressure in it. I think it has a leak, but not sure where so I'll use a leak detector for that. What could be wrong with the compressor noise? Can I just replace the compressor or do I need to replace the dryer as well, in other words, what needs replacing?

Summary of things I noticed when I charged the system the first time:
1. The system charged normally, but the freon seemed to leak back into the low pressure side (hissing noise in compressor) after the system is disengaged bringing the pressure way up.
2. The rear a/c is colder than the front a/c.
3. The compressor made a loud whirring noise when engaged (like an airplane). The more freon in system, the louder the noise.
4. The low pressure switch didn't work, so I had to bypass it to charge system by grounding out the leads to it.
5. Most of the freon leaked out in about a month. Suspect it leaked through the compressor.
6. Any advice is appreciated!!!!!