Power brakes problem?

[kynimrod]

I have been struggling with a power brake problem now for a long while, so I thought I might put it out here and see if anyone can help solve it.
1968 C10 50th anniversary edition with power drum brakes. When I first start and drive the truck, the first couple of times I stop I can really feel the power brakes. As I continue to drive it, the brakes get really hard(no more power assist). I replaced the booster and check valve last year and it made no difference. I checked the vacuum and it is 18 and idle and goes up some when rpm increases. I have adjusted, cleaned and checked brakes on all 4 wheels.

What is my problem? The truck came with power brakes!
Anyone shed some light on this?