No brakes, please help!

[xrunnr]

I am working on a 1974 109 ex-mod with dual brake master cylinder. So far I have replaced the cylinders at each wheel as well as the brake shoes and had the drums turned. After that I replaced the servo unit and rebuilt the dual master cylinder. I also inspected the brake liines but did not replace them. Everything was put back together and I bleed the system. I went around the truck 3 times at each cylinder and bleed them until there was no air left in the lines. At this point I tried the brake pedal but I got nothing. You can push the pedal down and it will either stay on the floor or come about halfway back up but either way the brakes dont work. With the truck turned off the pedal will go all the way down and come back up but with the truck running and the pedal is pushed down it stays down.
Does anyone know why my brakes do not work or why the pedal will not come back up?

[SafeAirOne]

Are the resivoirs staying full or going down as you keep attempting to get the brakes to work?

Is there any external leakage at any wheel cylinder or master cylinder?

What is the condition of the flex hoses--Did you replace those as well?

How about the anti-backflow valve (or whatever the English name for a check valve is)--Did you replace that?

Did you reassemble the MC innards correctly?

[xrunnr]

Yes, the resivoirs are staying full as we attempt to get the brakes to work. The only time the brake fluid level goes down is when the brakes are bleed.


there are not leaks at the wheel cylinders or at the master cylinder

hoses were not replaced but appear to be in good condition, no leaks

I have not replaced the check valve on the engine. Is that the check valve you are refering to? is there another on the brakes assembly?

Yes, the master cylinder innards have been replaced correctly. Before it was put back on the truck we tested it to make sure it worked correctly.

[jp-]

It's very important to have the brake shoes correctly adjusted when bleeding the system. If you don't have them adjusted when bleeding, the brake springs pull back in the wheel cylinders and negate the effort of each pump that you are trying to bleed out. This can make it seem like you are getting nowhere (and you aren't). Brakes should be almost touching the drums when bleeding. A good way would be to adjust the drums so that they are just at the verge of not turning when bleeding, and then back off after the job is done.

Double check your brake routing to the master cylinder. Also make sure that your pedal stroke is correctly adjusted to your master cylinder rod. If not correct, you may be moving the pedal a full stroke, but moving the MC rod only a half stroke, so make sure.