My 90 350 SDL has been converted to 134. Everything was fine for years but now at 70 degrees ambient temp my high side pressure is 150 psi. The low side is about 30 psi but after it runs awhile the AC stops working, High side is still 150 but the low side is 70 psi. Is there a relief valve bad? Or, what could the problem be?

it sounds like the compressor is failing. 150 is not much, and 30 low, is nearly right. what's the temp up there? today it was low 80-s little humidity. you're up in the "mountains" but should be similar temps.
you might just be a tad low on refrigerant, I'd need line temps, and vent temps, and outdoor temps, etc to fully diagnose the issue.

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

put more 134 in. at 80 degrees the high side stayed 150 the low side up to 40 AC is cool but as the car warms up AC quits. The high side still 150 but the low side is way high 70+.