I’m having some brake issues that are making me loose my mind. So here goes ... my rear brakes are not hooked up. They are plugged at the T that would feed the rear lines over the rear axle. I have put a new booster on a new master cylinder new calipers on the front. I’ve bled the system so much I’d almost pay someone to do it for me now. I have an offenhauser intake and have everything hooked up the way all the write up’s say to. My problem being I can’t get the brakes to hold pressure. I can get them pumped up eventually but once you release pressure the next time you hit them they go to the floor until you pump them back up. Am I missing something in this system or just so aggravated I’m over looking the obvious?? Also when you pump the brakes at an idle it changes the idle like vacuum is back feeding or something. If everything wasn’t replaced with new in the last 48 hrs it may not aggravate me so much. I’ve gone over all the lines inch by inch multiple times. Nothing is leaking and the little bit of rubber lines it does have are not swelling. I’m at a complete loss here and any and all suggestions are greatly appreciated

 

I am no brake pro by any stretch, far from it, but:
You said you replace the MC. Did you "bench bleed" it before you put it in? I know you need to do that so there's no air trapped in it. Essentially, you pull the line off the outlet port, press the pedal down, block the outlet port with a finger, release the pedal, lather, rinse, repeat. 3-5 times.
THEN go and bleed the brakes, right rear, left rear, right front, left front, LPSV.

Did you make sure the rear brakes are adjusted correctly? That little star wheel inside that moves the shoes in and out. I believe the rear brakes should be set so that they're JUST starting to drag when you spin the tire. I may be wrong on that, though.

Are you sure the O Rings on the MC piston are installed properly, and didn't get rolled out of their slots (whatever they're called) during installation?

Did you set the pedal height and MC pushrod before you started bleeding?

These are just things that pop into my little pea-brain right off the bat. I'm sure someone on here will come up with a secret I don't know and/or am missing, though.
Good luck to ya!
Pat☺

 

We're you having this problem before you replaced all the parts? Why are the rear line blocked off? Only way I've ever bench bled a master was using one of these - https://shop.advanceautoparts.com/p/...cb/20352246-p?

 

Did you bleed the rear line to where it's blocked off?
Even if you get the pedal to hold pressure you are still driving a death trap having your rear brakes blocked off. If you have any issue with the front brakes and you have.... NO BRAKES.
If you have an automatic your rear wheels will continue to push you along, on ice the front wheels will lock up and you still keep going.
Don't put everyone else on the road at risk, fix it right!

 

You have NO REAR BRAKES?!?!?

I would not drive your truck at all until you have your rear brakes. Yes, the front brakes do the majority of the braking, but that doesn't mean the rear brakes are not important!

 

The rear brakes are blocked because it has my roll around rear end in it while I rebuild the rear end that goes in it and put the disc brake conversion on it. I did not have this problem before but this is also a truck that hasn’t been a daily driver in years so some speed bumps were expected. Everything in the brake system is new brake booster master cylinder and calipers. I have since warrantied the brand new master cylinder and put another new one in it and I now have working brakes again. Appears something from the factory was not QC before it was sent out and yes I bled every part of the system as it should have been.