Air conditioning not cooling well

I have a 1990 Suburban that blows only slightly cool air on the front vents and alittle cooler air out of the rear seat vents. It is an r-12 system. Last summer it did not perform as well as it had all the years before, but was tolerable. This year is just isn't working right. I hooked a set of gauges to it and the low side is running around 50-60 psi and the high side is running about 120 psi. It had a short cycling rate as well about 5 seconds engaged and then kicks out about that long during idling. The short cyling made me think maybe it was low on refrigerant even though the gauge readings suggested otherwise, but thought I'd give it a shot anyway. I had added a can of refrigerant but made no difference. I'm stumped on this and an my son's grand prix a/c that I will post in that area also. Any help on these would be greatly appreciated. thanks everyone!

I have been studying a haynes a/c manual and have gotten a few ideas but still need some assistance. Exactly where on this system is the Orifice Tube? Any help on this would be greatly appreciated!

I have been studying a haynes a/c manual and have gotten a few ideas but still need some assistance. Exactly where on this system is the Orifice Tube? Any help on this would be greatly appreciated!

The orifice tube is inside one of the hardlines between the evaporator on the condensor. There are 2 nuts on the hardline where it comes apart and the orifice tube is inside there.

I have a 90 GMC K1500 Sierra and the orifice tube is near the battery on the hardline going into the firewall (to the evaporator).

Note that in order to replace the orifice tube, you must (by law) have the r12 removed from the system.

//2000CAYukon

thanks abunch for the info! I wasn't sure where to even begin to look for that!