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I had the same issue on my 74. Turns out the "Y" block at the rear of the truck was plugged with crud and wouldn't let any fluid pass. I replaced that connection and re bled the brakes until fluid ran clear. Also had plugged bleeders at the front of the truck. When we bleed the fluid and replace we actually end up freeing up gunk off of the lines and such and it gets into all of the places it can plug. The proportioning valve is at the front of the truck. Follow the lines from the master cylinder and you should be able to find it. If you are getting fluid out the front though my bet is on the splitter, ("Y") connection at the back. One line runs to the back and gets split between the two rear wheels. good luck. Hope this helps. Oh I also bought a reasonably priced bleeder setup that has the equivalent of a bug sprayer and plate to put on the master cylinder. Makes bleeding so much easier....If you are interested I will see if I can find a link....ricky