[cypress TX 72]

Shane,
If the low side is shooting up past 50 psi and the compressor is engaging - then either the compressor is not pumping or the refrigerant is not flowing. Think of it this way - the high side of the compressor is freon leaving and on the low side its comming back from the evaporator - so if the pressures are equaling out - then it is not being condensed.
When you say you cannot use the orfice tube-does that mean the old one out of the 72 or the new one- because it has to have one to work and it has to be the correct metering for that set up. (or close, it doesnt have to be exact-but it has to have one close)

Gauges are going to be a big help for you to try and pin point the problem
but
If you have a metering devise such as a orfice tube or cap tube in place then the first thing I would try is to dump some of that 134 and see if the pressures go down - while running the truck.
Also, we need to see if the condenser is getting hot, and/or is the suction line (the big line) comming from the accumulator is getting cold.
If the suction line is not getting cold - then I would lead to say that
1.you have either lost your orfice tube or blown it out
2.you have a restriction line the system somewhere
or
3.that there is no orfice tube and the system is just pumping around without metering.

Listen I know it sucks, but if you want to try and fix this yourself I would say that guages are going to be a must- do you have a buddy that may loan a set to you? Cause I am sure after all that the last thing you want to do is have to buy a set of freaken guages.
Man, if you where here, i'd come over and help you.- my wife and kids are away
T