71 C20 Power Disc brake trouble

[Don Brace]

Put new calipers on the front, rebuilt the rear cylinders and installed new master cylinder. Had my wife help me bleed the brakes and have no air in the lines. The pedal is not soft. When I depress the pedal it goes almost to the end of the travel. The brakes stop the truck but the pedal goes almost the full travel. If I push hard on the pedal it feels like the pedal hits a mechaical stop. I think this is the end of the travel for the master cylinder. How much travel should the pedal have? Mine is about six inches. It does not go to the floor. I have put on two new master cylinders. I checked the adjustment of the rod from the booster that operates the master cylinder and that is correct. There ia about a half inch free play in the pedal.
Thanks for any help.
Don

[ERASER5]

Did you bench bleed the MC before installing it? This step cannot be skipped.

[Don Brace]

Eraser5 thanks for the reply.
Yes I did bench blead the MC.
How do I check the prop valve to be sure it is working right?

[hvychve]

Nothing wrong with tracing your steps, I would verify your wife is holding firm on brake pedal while you crack bleeder, if she is doing so, than I would verify you have good adjustment on the rear brakes, if adjusted good and you bled entire system including master and have the correct rod from master to booster, you should have good pedal. a bad booster will normaly just give you no assist when running. what sounds like you have is air in the system, sorry, I know you said you have no air, but sure sounds like it.
try gravity bleeding for a while, open the right rear bleeder and let the fluid push out any air, leave it open for a while, than close it and do the other rear, than do rt. front and finaly lt rear, making sure you keep resevoir full of fluid, if it goes dry, well, you have to do it all over again. than after gravity bleeding you can have her get her strong legs up there and hold the brake pedal down while you crack bleeder , noting if any air or bubbles come out, do each wheel a number of times, if you have air up towards the master , you will have to bleed several times to push out if its in the rear lines.......

now if its not air, like you say you are pretty sure it isnt, and probably mad by now, due to fact I say it sounds like it is.........

are your calipers on the correct side, bleeders should be on the very top , so air can be removed entirely, if they are switched from side to side they will have the bleeders too low to escape all the air..

No other leaks , like rear rubber hose or any of the front hoses?

well, I cant think of much more, but good luck and keep us all informed.

[Don Brace]

I will give the gravity bleading a try after work tonight. I will also check the adjustment of the rear brakes.
The complete brake pedal assembly and the booster is from my '69 C20 3/4 ton that I parted out. I think the parts are the same for a '71 C20 1 ton. Thanks for the ideas. I am not mad just annoyed that I cna't seem to get them to work. This truck has been the project from hell.

[Don Brace]

Today I adjusted the rear brakes. They were a little loose. No help. I drove the truck a little today and tried several panic stops on the wet street. The peddle would bottom out and not lock the brakes. I think I will take the truck on my next day off which is Friday and have them power bled.
Don Brace MN

[Don Brace]

After replacking the master cylindar and bleeding the brakes with no improvement I decided to change the power booster. That fixed the problem.
The brakes now work like they should. The that connects the booster to the master cylinder was loose and could be pulled out of the booster. The new booster the rod was tight. Thanks for all of your help.