Follow Joe's instructions to check things. Also, you need to flush your brake system. The industry recommends annually, I recommend every other year for most vehicles. Regular brake fluid absorbs water and is easily contaminated, probably why you have a problem.
If the truck sets all winter every year, you might want to convert to DOT 5 fluid, silicone base. It doesn't absorb water, but is a little tougher to bleed and not quite as responsive as DOT 3. Biggest issue with switching is you have to replace all rubber in the system, hoses and cylinders.
The proportioning valves are sold by some of the vendors. Bought mine from GMC Pauls, but there are others. Go to the links section and look around.
