air conditioner help please...

I put a 5.0FI 5spd 93 GT engine in my 1970 Mustang with factory air - I replaced the condensor, expansion valve, dryer, lines, etc... It was charged with 134 freon and got cold but started popping the high pressure relief valve - lowered the pressure and stopped the popping but not cooling now. Now there is envirosafe freon in the system (runs appx same pressures as 134) and I can get it to cool a bit but not getting cold. I checked the pressure on the low side - compressor not running 85-90psi drops to 0psi in 5 seconds once it is running - it should be dropping to 40psi when the compressor kicks in - I have ran it like this for 50 miles at highway speed and nothing has changed - any ideas on what I can do to make it go cold. I have added another can of envirosafe and the pressure on the low side goes well over 100psi when the compressor is not turning and down to 0psi in 5 seconds when the compressor is turning - By the way the hot water diverter valve is working, the vacuum doors all work properly, the compressor is kicking on and the clutch is working fine etc...

Thanks for the responses guys - I have 2.5 12oz cans of 134a in there now but no dice - same thing going on - I do not have a high pressure guage - just a low pressure one - it will max out the guage when the compressor is off and when I turn it on the guage will drop to 0psi in 5 seconds - I see we are all in agreement that there is some sort of blockage in the system - all new lines, new expansion valve, new dryer, new condensor, used compressor and original evaporator under the dash - at this point I think I will take your insight and go to a qualified a/c mechanic and have the system vacuumed down to -30lbs and have them add a cleaner to the system and see about clearing the lines of any blockages by blowing them out etc... and letting a professional charge the system once this is done - I just hate thinking about someone else working on my car.

Air conditioning FIXED now - I ended up taking it to a shop - they located the problem - the NEW expansion valve was defective and I got another one and a new dryer to boot - they flushed the system, vacuumed it and re-charged it with 24oz of 134 freon, proper system oil, etc... pressures are supposed to be 20psi low side at 85 degrees and 175-200psi high side at 85 degrees and car at idle. As far as the question about orfice tube style compressor being mixed with the expansion valve - they work fine - you have to use one or the other - I was told it was easier to stay with the expansion valve system and just make up the hoses to fit the newer 5.0 style (F6 I think is is called) compressor. Hope this helps someone else...