Stumped with brakes

[beyerku]

I can't figure out my brake problem. 1975 k20 with vacuum booster.

I replaced all the brake lines and the proportioning valve and couldn’t get the brakes to ever firm up. I then replaced the master cylinder and it seems like the rear compartment (for the front brakes) is sucking in air. I have no leaks and no drips and of course, the master cylinder is brand new. We bench bled the master until there was no air, but when we start to try to bleed the brakes, it seems like the rear compartment is pulling in air. I linked a video below of what happens after you let off the brake pedal when we are trying to bleed. Both front and rear compartments will push a little fluid up, but when you let off the pedal the rear compartment bubbles.

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Is this a bad master cylinder? Is it possible for the master to pull air bubbles all the way from the proportioning valve? We are stumped...

For additional confusion, we can bench bleed the master cylinder, and then put the master cylinder on the truck with the bench bleed hoses still in and then the same air problem occurs. I have a crazy theory that air is being pushed into the master cylinder in the rear through the vacuum booster because for some reason, the master cylinder is tight enough to the booster that when depressing the brakes air is pulled in, but when the brake pedal is released, the air can’t escape and gets driven into the master cylinder. I assume this shouldn’t be possible and would also be evidence that my brand new master cylinder is bad. Help!

 

[ali_c20]

I have seen brake fluid sucked in the booster but that bubbling is strange.
Is there a seal between master cylinder an booster? If no I don't think that master/booster can be air tight.

Are only the rear brakes not working properly?
Is the bubbling occuring with vacuum applied to the booster ?

Try gravity bleeding the brakes, takes some time but works.

The brake system pressure is really high, if the problem should be below the master there would be a leak.

Most of the parts for our trucks are made in China or elsewhere and quality has gone south a lot. Unfortunately it's common to get a new part that's broken out of the box.

Get a new master, if the problem persists you have to continue trouble shooting, but hey you have at least a spare master.

 

[Oliver]

Did you replace the flex brake line to the rear axel? If you have a load sensor for the rear brakes, is it possible it is sucking air?

 

[hack_man]

This reminds me of the time I had pulsation in my brake pedal so figured I had warped rotors. I replaced the rotors with brand new ones and the pulsation got worse. Went back for more rotors and fixed it.

Sounds like a bad new MC

 

[1975K20REBUILD]

Did you get the bubble problem fixed?