1968 gmc truck brake problems

Does the reservoir fill to the top when their bleeder valve is open? You probably just need to bleed the system again (and if it was low enough, the main cylinder too). Close all bleeder valves but the furthest wheel from the master cylinder and begin to add fluid. Have someone pump the brakes when doing this. When fluid (and no bubbles) is coming out, close the valve and move to the next furthest wheel. Repeat for all wheels until done and top off the master cylinder to the "full" line. If that does not work and both front or both rear brakes don't function, you may have a blockage in the master's regulator that needs to be corrected.

Thanks for the advice. I got the rears to work and adjusted them then i moved to the front. I replaced all the wheel cylinders and found on the front the rubber lines to the cylinders were collapsed. The cylinders were also froze. I had to clean out the master it was filled with grim. Thanks again

I had same issue mine was back wheel cylinder was sucking air try clamping off all cylinders with clamps on rubber lines take one off test pedal repeat till you find your problem helps if you have someone to assist you.