bleeding the brakes problems

my dad and I put a new master cylinder on the m-715 and then we filled it up and then we unscrewed the bleeder bolt put the might vac on it and try to pump the brakes but no fluid came out

When you say bleeder bolt are you referring to one of the bleeder screws at the wheel cylinders or something else?

Did you bench bleed the master cylinder before install? If not, it wont flow to the rest of the system...if you did and the master is good...then you should be able to have 1 person step on the brake pedal and the other person then open a bleeder screw on a wheel cylinder and get pressure out of it. If the system is really low on fluid, it might take a couple tries to get fluid all the way to the end of the line...

If you are trying to bleed and no fluid ever gets to the wheel cylinder, then you probably have to replace the flexible brake hoses as over many years of use, they can swell shut making fluid flow through them impossible. I would recommend changing all 3 of them if that is the case....one at each front wheel and one from the frame to the rear axle out back.

If something else is holding up the show, please post and I will be glad to help more...

Might be a good time to flush some clean fluid through the whole brake system anyway to make sure everything is ship shape going forward...these brake systems are known for having crud built up in them, especially due to these trucks being 50+ years old and spending most of that time sitting...

what jon said. If you didn't bleed the air from the master cylinder, you should start there.

The military used pressure bleeder systems. The screw on master cylinder caps make it so easy too. A garden spray rig can very cheaply be made into one. Or you can buy something like this from these people:

https://www.motiveproducts.com/

I have had pressure bleeders push fluid through just fine when vacuum bleeders can't get any liquid to the corners. That is why I posted this. They just work better in my opinion. Plus, as already pointed out. Your brake hoses and almost for sure your wheel cylinders are probably gunked up and need to be cleaned out or replaced.

If you can't get fluid from the MC to the wheel cylinders, you need to check the rubber lines. If they are old, they can swell or break apart on the inside plugging the line.

Start at the MC and work your way through the system. Crack the lines at the MC and push the brake peddle and see if you get fluid then go down the lines cracking the flare nut at each connection and see if you get fluid. This way you will be able to go through each part of the system and see if you have a blockage etc. Also check the push rod on the MC to see if it is actually pushing the valve in or not. I had that one time..drove myself crazy trying to figure out why I had no fluid only to find out I wasn't actually pushing the piston in. Good luck with your system.

And when you crack the line as suggested, make sure that you close/tighten the line before the helper lets up on the pedal.