A/C not very cold

I have a 88 Suburban with front and rear air. Last summer the A/C did fine. This summer it take a while to get cool, but it will not get very cold, especially when idling. I have 45 psi low side when it is 75 degrees and 70 psi low side when it is around 90 degrees outside. Even so it still is not very cold. Any help? It is r-134a, and there is no stop leak crap in the system.

First off, thank you for your service to our country (US).

Secondly, have you put an actual manifold gauge on both the low and high side, or just one of those hoses with a gauge on the low side?

If you have run a manifold gauge on both ports, what is the high side reading?

Another thing to check for is crud built up around the evaporator coil, crud on the condensor, etc. Is the compressor cycling - i.e. are you getting your low side reading about 30 seconds after the compressor kicks on? Sometimes, I've noticed with my R134a conversion on my '88 Caprice, that a static reading on the low side can be well over 70 pounds, but once it kicks in and stays running for a bit, it tends to drop down to around 30 PSI. If it's low, the compressor will cycle even if one of those cheap gauges on the low side reads in the red - that's why the manifold gauge is needed, to see what's happening on the high side. The high side pressure should roughly be 2.5 times the outside temperature (i.e. 100 degrees outside you should have 250 PSI etc).

Thank you for your appreciation. Second, I tested the low side with just the cheap gauge that is on a recharge hose. I test it only on Max A/C so the compressor is working at all times. When I obtained the 70 psi reading, it was after running the truck for around 6 hours, I use it for work, and it was a hot day. I obtained the 45 psi reading last night after running the truck for a couple hours, it was around 75 degrees outside. I will buy a manifold guage kit and see what the high side is doing. I will look at the evaporator and see if it is dirty. The rear A/C was on both times as well. I did look in an A/C repair manual and it stated the low side should read 30 psi in 75 degree weather. Will the extra pressure cause higher vent temperatures?