Here are a couple of sites that I had searched earlier on this topic. This one Heater Control Valve - someone who rebuilds and sells valves, I was going to contact him and talk, never did.

The second is instructions, albeit for a different valve thatn the Chevy Truck, but seemed to be worthwhile heater control valve repair .

Good Luck!
Mike

My old truck's still running good, my ticker's ticking like they say it should, I got supper in the oven, a good woman's loving, and even my bad days aint that bad...Yeah, I'm a lucky man!
1953 Chevy 3100 Shortbed

1965 C-30 Panel Truck
1963 C-20 Long/Fleet Pick-up California Black Plate