Question about bleeding brakes

[western star]

71 with a dual master cylinder. I replaced the brake line that runs form the junction block on the rear end to the drivers side rear. I did have someone pumping the brakes and holding pressure on the brake pedal. The first time I cracked the bleeder I got air. From that point on all I get is a trickle of fluid. After trying this for quite awhile a someone suggested that I might have a saftey valve in the brake lines that prevents all brakes from failing in case of a bad leak therefore I'm getting mostly front brakes only right now. All you have to do is reset it. Great but I have found no valve but there is what apperars to be a button on the master cylinder with a rubber boot on it. . I thought I would ask before proceeding. Any help would be appreciated.

[Grim Reaper]

Here is a couple things to look at.

First let me explain how a twin master works.
The rear piston pushes the front. If there is air in the rear circuit then the rear piston will bottom out and push the front. In normal conditons the fluid bewteen the rear piston and the front is what pushes the front. This is why it it critical to bench bleed the master BEFORE installing it on the truck. If the master is not level air will be trapped in it and very hard to get out with normal bleeding.

Next problem. Did you install a Combination valve (Combination valve is the correct term, It has a proportioning valve for the rear circuit and a hold off valve for the front on Disc/drum set ups) for a dual master? If so they can be a booger to bleed. The inlets from the master are on the top and on most the rear circuit connection is on the side or on the bottom. When pump bleeding the air will compress and the Proportioning valve for the rear circuit will close and not allow fluid to make to the rear brakes.

That sounds like your problem. What I have done is cracking the line loose coming from the rear circuit from the master to the Combination valve. get it to where it's leaking. This will cause the master to gravity bleed so make sure it doesn't run out of fluid. Tap on the Combination valve with a wrench to try to dislodge any bubbles and they should burp out.

Tighten the line, refill the master. See if it will gravity bleed the rear circuit by just cracking open the rear bleeders.

[western star]

Grim Reaper. Thank you for your response. There is a junction block on the front of the rear end. It is only a block to tie in the lines. All I did is replace the line from that block to the drivers rear wheel. It makes sense what you have told me and I will try this. I did not install a comination valve. However I probably do have air in the system from the master cylinder as the rear part of the master was quite dry when we added the fluid after installing the line. The first couple of trys at bleeding seem to be working well and then it stopped to no air and a trickle of fluid. Someone was saying to be very gentle with the pedal pumping and do not push hard on it when holding down. I will try and follow your instuction to correct this problem. Thank you

[ddsmith]

Another thing I've found that helps is to crack the fitting on the line you put in. That air in the line is a pain to get though and out. If you crack the fitting it doesn't have to make it out through the bleeder. Do the same gravity bleed thing. That way you don't suck air back into the line.

[lukecp]

I had a similair problem a few months ago, turned out my master cylinder wasn't pushing very much fluid to the rear brakes, and needed to be replaced. I replaced the MC and it squireted out the fluid, instead of it just trickling out.