Toyota Repair: A/C Slowly gets warmer, toyota sr5, center vent

Question
Why does the A/C temperature in my 1986 Toyota SR5, 22REC, X-cab truck start at 38 (on recric from center vent), then after about an hour of continous use begins to rises to 50 degrees even while driving the inerstate at 70MPH in 88 degree tempeature? Never did that before. I think it would continue to climb if i drove it longer. The system is stock (R12) except for a new A/C compressor clutch I put on last summer. When it was new 20 years ago it came out 35 degrees.

Need help. The hot weather is soon to hit. Thanks

Answer
There is dirt or moisture in the system and as the a/c runs the expansion valve gets stuck and cant work properly.
Have the system evacuated, replace the reciver dryer and recharge.
If you can have them put the R12 they took out back in. Works better and you dont have to convert to R134a



Mike