Brake pedal jumping back up?

[Vinnie1511]

I recently purchased a 1970 m35a2, I was going through the braking system and I replaced the master cylinder. After that I bled from the m/c to the air pack and then bled each wheel. After completing the bleeding process when you go to press the brake pedal it’s stiff as a rock and I hardly have any braking power. I noticed when I press the brake pedal down (hard) the air pack kicks the air on and then the pedal pushed back torwards me. I oiled the air pack also with no help? Any idea what could cause the pedal to be so stiff and to jump back at me once the air pac engages? Thanks for any input!

 

[LowTech]

I would guess that its the airpack bleeding past the cup? You might have to join the rest of us that just rebuilt our airpacks.
Hopefully someone that does lots more work on these systems than I will be able to give you some input as well.

 

[Vinnie1511]

Thanks for your reply I was thinking about replacing it just wanted some input before I pulled the trigger, gonna pull the wheels off also and make sure all the wheel cylinders are moving and operating.

 

[LowTech]

I don't think I would pull the drums off to test that. I'm pretty sure you can over extend the wheel cylinders if there's no drum to stop them.
Just jack the wheel off the ground and spin it, then have someone push on the break pedal while you're spinning it. Wheels chocked and everything in neutral.
For the rears you'd need to lift both wheels on that side or they won't spin.

 

[Vinnie1511]

I would guess that its the airpack bleeding past the cup? You might have to join the rest of us that just rebuilt our airpacks.
Hopefully someone that does lots more work on these systems than I will be able to give you some input as well.

Found the problem if you were curious, ordering a rebuild kit for it. Other then the broken spring and messed up piston seal everything else is clean no rust or pitting throughout

 

[LowTech]

Good job! Thanks for posting up your findings.
So it was air blow by. Nice to know it causes that result!