Mike,
I have a few suggestions before you try to convert your truck to manual brakes.
You mentioned you have low vaccuum at idle. What type of engine are you running. Is the connection on the carb good? Have you checked your intake for any leaks? How about the carb? Since you have replaced a good amount of components you need buddy to help you. Turn on your truck and have your buddy step on the brakes. Check for air leaks around the master cyliner vaccuum line connection. You won't find a leak there unless a buddy helps you. My brother was fighting a brake problem for a while and it was not until I help him with the above proceedure that we found the problem.
Do calipers work on the front? Remove the calipers and step on the brakes. They should move freely. How about the back? If you remove the tire can you remove the drum easily? If so then you need to adjust your brakes. Do a good job bleeding the brakes with a buddy. Brake fluid is cheap bleed the entire lines very good.
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Gerardo
1983 Custom Truck
TX Plates: "BAD BOW"
http://www.geocities.com/abetterchemist/