Heater core heating up A/C evaporator coil???

[Greenlee]

I have spent a small fortune in the last year replacing just about every component of the A/C system on my 76'. New compresor, new lines, new dryer, etc. However, I still don't think it blows as cold as it should. On a hot day in Houston I have to leave it on high and it's just a little more comfortable than driving with the windows down.

This week I had it checked by a couple different A/C places who told me it was full of freon and should be working fine. The last guy I took it to had a theory that the hearter core is heating up the evaporator coil. He said they are in close proximity to one another and the doors that control air flow from the heater or A/C probably aren't opening and closing like they should on a truck this old. Another guy told me something similar about a year ago and said he put a valve in the line to the heater core to bypass the heater on his Chevelle.

My question is, has anybody else had this problem? Also, what is the best solution for the problem? I put a valve in the line from the radiator to the heater core last night, and drove it to work this morning and didn't notice much of a difference with that line blocked. Did I put the valve on the right line? Do I also need to block the line from the heater core to the engine?