i have a 77 highboy, and i have been having problems bleeding brakes. can someone tell me how the proportioning valve works, with the pin in the front. i cant get fluid to go to the rear lines with the line connected to the valve.

 

OK i had the same problem and it was a real headache! It's 1 of 2 things.(hopefully for you the 1st)If the pin is in all the way you must pull it out to bleed the rear (it'll stay out and move w/the brakes when there's some rear line presure built up) the second and what was wrong w/mine can accure on neglected systems where the fluid is very dirty. The P valve must be removed and completly taken apart to clean out the valves and seals of whatever junk is blocking it. To check which passages are blocked I used comp. air and a rag.
Good luck, I tryed and gave up about 3xs before i finally got it.