Air Cond - Help?

Ok you ac guys, I need some opinions. I have an ac and ref. background, residential and comercial, not auto.
Here's what I got.
Underdash ac in my 66 chevy pu. All new hoses, condencer, drier, and Sanden 508 compressor. Charged to 40psi, low side 230-250psi, high side. At idle of aprox 850rpm. At 2000 rpm drops to low 20's and up to about 275 to 290psi high. R134a. Expansion valve system. But my temp drop across evap coil is only 20deg give or take. I have played with dropping some of the charge and it might be slightly better but just doesn't seam to cool as well as it should. I didn't have a vac pump toevacuate system, so with an all new system i just purged it good, reverse direction to remove air. The pressures are fairly steady and not fluctulating as would with a central unit when air was present. Any Ideas?:confused:

I wish I could help But I just don't know, Sorry.
Just give some of the other guys some time to see it.
I am sure one of them will know what to do.
Kurt

If this is recent in Texas I'd agree it sounds like a low charge. Your 40psig low side correlates to about 45F ambient, and I'd be surprised if you don't need to be in the 80/90 psig range, according to the chart.
http://www.csgnetwork.com/r134apresstempconv.html