need brake help

[vardenafil]

i have a 1972 k20 pickup. The brakes never worked from the get go. The previous owner stated that the brakes just need a new rear cylinder. but if the fix was that simple he would of fixed it before he sold it. when the truck is off the pedal is low but hardish. it doesnt get hard until the bottom of the pedal stroke. soon as i turn the truck on the pedal drops all the way to the floor. there is no stopping power what so ever. when i turn the truck off the pedal pushes back against my foot several inches.

since owning the truck i have done the following brake wise.
- replaced front and rear calipers
- replaced all rubber lines. switched to braided steel lines.
- replaced the master with a disc/disc master.
- replaced the proportioning valve with a disk disk master.
- bled the truck using numerous times using different methods.
- checked to make sure the pedal attachment is in the right hole.

im at a loss at what to check for next. the only thing i havent replaced is the booster. but if the booster was bad the pedal would be hard not soft and spongy right? Any ideas what to try next?

[Rust_never_sleeps]

Sounds like a pedal height problem. Too much travel before you start putting pressure in the system, and you're running out of travel before you bottom out the MC.
Is the rod/rod gap correct between booster and MC?
If so, the pedal height adjuster between the pedal lever and the booster could be way too short.

Booster failure usually provides no boost, so a hard pedal and no drop with engine vacuum, you are correct. Seems like yours is functional

[Sheepdip]

As Rust Never Sleeps says, Is the push rod out of the booster into the back of the master cylinder the correct length?

Not all M/C's are the same depth in the rear. That's a simple check with a caliper or contact putty.

[vardenafil]

I’ll check the rod length this weekend. Hopefully that’s the issue because that’s an easy fix

[vardenafil]

ok so i checked the rod length and it seemed ok. i pressure bled with my motive bleeder and i out a quart through each wheel. i have a decent pedal now. i cant panic stop with the truck nut it stops good. the rear wheels lock up if i smash the pedal too hard. i may need a adjustable valve for the rears. i did notice that one of the rear calipers from lug nut 4x4 is funky. the caliper is supposed to spin one way to adjust and the opposite way to set the parking brake. the drivers caliper i can adjust the arm to wear it barely moves against the stop. the passenger rear caliper the arm keeps spinning it does not have a catch to it. it spins both ways. so i think lug nut gave me a defective caliper? or am i doing something wrong?