Need help no brakes

[Noah96]

Ok everyone having brake issues. Its a 1969 C2500 4speed GMC with vacuum boost. I've replaced the back brakes and springs along with 1 bad rear wheel cylinder. On the front I've replaced both wheel cylinders along with the Correct Master cylinder which was bench bled before installing. Now I completely bled the system from farthest corner forward go the air out of the lines. When the engine is not running my pedal feels good but once I start the engine the pedal goes to the floor, but with the truck running and the vacuum hose disconnected the pedal is good when i'm in neutral then once I put it in gear I have no pedal.
Also Ive checked for leaks in my brake line and nothing along with the backing plates no sign of a leak.
Any help would be appreciated I am stumped ask a few local shade tree mechanics and no luck.

[sick472]

Alright Noah, I'm no brake expert, but your not getting much help so I'll try.

If, in any situation, the pedal goes to the floor and you do NOT have any air in the system, the brake fluid has to be going somewhere. If you have no visible leaks, I would suspect that your booster is taking on the fluid. I would check your fluid level, make note of it's actual level and cycle the brakes like you described by causing the pedal to go to the floor. After several cycles, you should see the fluid level lower than before. If nothing in running down the lines anywhere or hitting the floor...I would suspect the booster.

IF the level does not change...look into adjusting the booster push rod. Some reading on that subject may help you out.

[AussieinNC]

1/ Check the metal brake line in the chassis rail, passenger side that runs to the rear. They corrode and pin hole...
2/ Remove the master cylinder from the booster and look inside the bore of the booster for brake fluid....sometimes the booster seals will suck fluid back out of the master if the master piston is not returning fully.
3/ Also, check the metal lines around the clutch linkage pivot...I have seen strange wear patterns some times on brake lines...maybe one has worn thru...
4/ How are you bleeding the system? With a pressure bleeder or by pumping up the pedal and having someone crack the nipple open? Its always best to use a pressure bleeder if possible.

5/ Why replace only one rear cylinder? Maybe the other old one is now leaking?

[Noah96]

Well I talked to some mechanics at my dads office and narrowed it down to the brakes not being adjusted after being put on. I have never delt with drums and didnt know I had to adjust them but it still could use some tweaking. Im learning how to wrench on this truck by the way. Also the last wheel cylinder was not leaking but I have plans to replace it soon when I have another nice day.

[Andy4639]

Brake shoes need to have them just scrubbing when you put the back drum on. You will have to do each one. There should be a star wheel inside the drum on the bottom of each brake. Adjust the wheel until the drum just does go on the studs.