1966 Fury soft brake pedal issue

[Canadian 66 Fury]

Hey everyone I am looking for some support on this as I am stumped. I have 4 drum brakes with booster, duel plane master cylinder from a B body Charger. Bled the brakes numerous times no air in the system, when you pump the brakes with the car off the pedal goes hard and holds pressure. If you pump the brakes and start the car the pedal drops slightly and holds. If you release and apply the brakes the pedal almost goes to the floor. At idle the brake will hold, but if you apply throttle at all it will over power the brakes easily.
The booster push rod is adjust properly and I am getting good vacuum in the booster and it is holding pressure and check valve tested good. Any insight on where to go from here would be much appreciated as I am lost in where to go next.

 

[57fury440]

Did this just start happening or did it start after some work was done? Are the brake shoes properly adjusted?

 

[Canadian 66 Fury]

All new brake parts with shoes properly adjust, Ebrake adjust brake have just been fully bled. I’ve notice it since I change master cylinder from the single to the duel but it has never been this bad. Master cylinder is from 69 b body drum drum setup.

 

[Canadian 66 Fury]

Did this just start happening or did it start after some work was done? Are the brake shoes properly adjusted?

All new brake parts with shoes properly adjust, Ebrake adjust brake have just been fully bled. I’ve notice it since I change master cylinder from the single to the duel but it has never been this bad. Master cylinder is from 69 b body drum drum setup.

 

[rags]

If you release and apply the brakes the pedal almost goes to the floor.

sounds like a bad master cyl. if you stab the pedal and it holds but falls away slowly to the floor under the lightest pedal pressure, that's a bad master.

 

[Big_John]

You still have some air in the system.

And "new" doesn't necessarily mean "good" anymore. I had a junk Centric wheel cylinder that would bleed just fine, but the bleeder had a leak that would pull air in, but not leak fluid out. Drove me nuts (well, it is a short drive at least) . The way I found it was to block off the master cylinder ports one at a time. I ended up finding some NORS brake cylinders on eBay that were actually pretty cheap. Worst case with those is you have to put new seals in, but that's nothing.

There are always some mixed reviews about buying from this guy, but his videos are really helpful.

 

[WOT440]

At idle the brake will hold, but if you apply throttle at all it will over power the brakes easily.

I had similar issue with brakes not holding, not locking up at low speed in garage. I also had just replaced, with new wheel cylinders and lines from Rock Auto.
Contact Craig, @mobileparts. Tell him you need a MC, front and rear wheel cylinders, and brake hoses. He will get you the parts to get the job done right.