rear brake problem

I have a 1998 dodge ram 4x4 5.9 liter. i kept replacing front rotors and brakes until i finally found out i wasnt getting brake fluid to the rear brakes.
i started cracking brake lines an found there was no brake fluid getting through the proportion valve to the rear brakes, i replaced it and it only lasted about a 2 months and then again no brake fluid so it was under warranty so i went down and got another it worked for a week no now brake fluid?
it is possible but rare to get 2 bad valves could there be another problem electonic? or something else? i have been working on this problem for some time now and at a total loss someone please help.
also the reason i ask electrical is that some time ago a mechanic told me my main computer might be going bad on another not related problem so i didnt know if the main computer could be messing with the proportion valve.

is the proportioning valve behind the gas tank?....in the frame rails

Sounds like you are not getting the brake system bled properly. you might consider taking it to a shop to have it bled or open all 4 bleeder valves and let the brake fluid gravity feed, be sure to kepp the master cylinder full at all times. If you have RWAL(ABS) than you might need to bleed the brakes differently.
Get a service manual subscription at www.alldatadiy.com for all the correct procedures.