Another brake thread.

First, I've got a 73 c20. It's 8lug, with discs in front, and drums in the rear. I've been at it all day. I bought the truck and knew there was a brake issue but the previous owner didn't know what was wrong. Brake pedal goes all the way to the floor. The first half its easy, then the second half it's hard like I'm doing a leg press. It barely stops.. BARELY
I'm also getting no fluid to the rear brakes.
I've bled brakes (passenger rear, driver rear, passenger driver, front driver) using the two person system. I've done a tube in a bottle until no more bubbles, and borrowed a vacuum bleeder. No change.
I used a master cylinder bleeder kit to bleed it. Ran lines returning to the fluid back to reservoir, and pumped until no more air.
Blocked the master cylinder off and blocked the 2 outlets. Pumped, and built pressure (only time I've got pressure). I opened one outlet, and the pressure escaped. Tried the other outlet, and same.
I disconnected the rear brake line from the proportioning valve. I ran air from the front of that, and went to the rear. No air, no bubbles, nothing, so I'm believing an issue or clog somewhere along the line?

I also ran fluid into the lines I disconnected from master cylinder and fluid runs out of the proportioning valve.
I don't know what more to do. At this point I'm thinking replace master, booster, and lines. I just didn't want to throw money at it if I could find the issue/issues. Thanks for all the help

Check this thread:

Still spongy (click)

Hard pedal does booster hold vacuum? Start engine for a few seconds. Pull check out of booster after a couple minutes. Should have at least some vacuum stored. No stopping power with a hard pedal could be frozen cylinders and calipers. They can still bleed ok but be stuck inside.

What happens if you block off the ends one at a time?

What parts been replaced so far, if any?

If fluid flows at each corner doubt there is a blockage.

Turbo4whl said:

Check this thread:

Still spongy (click)

Click to expand...

Thank you, I'll take a look.

fast 99 said:

Hard pedal does booster hold vacuum? Start engine for a few seconds. Pull check out of booster after a couple minutes. Should have at least some vacuum stored. No stopping power with a hard pedal could be frozen cylinders and calipers. They can still bleed ok but be stuck inside.

What happens if you block off the ends one at a time?

What parts been replaced so far, if any?

If fluid flows at each corner doubt there is a blockage.

Click to expand...

I don't know how to check the booster so I will do that next.

I blocked off the 2 outlets on the MC, and when I do that and pump the brakes I get pressure. Felt like typical pumping brakes in a car turned off.

I only replaced the brake pads and rotors.

I'm not getting any fluid to both rear. I checked the proportioning valve and I believe I got it centered. Last night I disconnected the rear line where it connects to the proportioning valve, and ran compressed air through, and still nothing out the rear bleeders. No air, no bubbles, nothing.

Not sure why a few here suspect the proportioning valve. In my 30 years as a line tech probably only replaced one or 2. The valve cannot restrict flow by being off center only. Getting plugged with crud yes, possible problem yes, but very remote.

Getting back on the subject, No fluid at the rear? Remove the bleeder make sure it is open, dirt can clog them. If nothing found, break the line loose at the cylinder. If fluid is present, needs cylinders. If not, keep going upstream until you find fluid.

fast 99 said:

Not sure why a few here suspect the proportioning valve. In my 30 years as a line tech probably only replaced one or 2. The valve cannot restrict flow by being off center only. Getting plugged with crud yes, possible problem yes, but very remote.

Getting back on the subject, No fluid at the rear? Remove the bleeder make sure it is open, dirt can clog them. If nothing found, break the line loose at the cylinder. If fluid is present, needs cylinders. If not, keep going upstream until you find fluid.

Click to expand...

I removed both rear bleeders and let sit overnight. Bone dry.

Looks like I'll be breaking every line from the rear up until I find the issue. Can lack of rear brakes cause non existent front brakes as well though?

One problem at a time. Try breaking the line loose where the hose attaches to the axle. If fluid leaks out either both lines across the back are bad/crushed or more likely cylinders are bad.

I'd replace all hoses first. Even if they look good on the outside. Hard lines seem like they're rarely the problem, but hoses sure can be.

Here's an update

Thanks to everyone that helped. Replaced the booster, and that fixed the hard brake pedal. No more standing up on the pedal to stop.
Continued to not build pressure though, so new master cylinder (yes I bench bled it) and now the truck stops.

My brake pedal still feels extremely low to the floor and very squishy, but stops. I've made a huge improvement. Next step, I need to address why no fluid is going to the rear brakes, and why my brake light inside is flickering on and off.

I'm going to make sure proportioning valve is centered, and replace the rubber hose in the rear and go from there following Fast and AZ instructions.

This MC is what NAPA said was for this 73 c20 3/4, but it's different. The outlets are switched so I had to gently nudge the brake lines to work. With that said, would I have issues, or should I look for a different MC?

Different MC can have different size bore, I'm thinking smaller the bore more pressure going to brakes but not sure.

Make sure master is correct for the vehicle. Could have had a wrong one on it? Larger line and reservoir goes to the front 1/2. Obviously, there is air in the rear 1/2. Need to fix that problem next. If the system is completely dry it may not self bleed. Get a helper and try bleeding it. Don't dwell on the brake pressure warning switch, it is working as designed. Fluid in the front 1/2 and none in the rear will turn the lamp on, differential pressure.

fast 99 said:

Make sure master is correct for the vehicle. Could have had a wrong one on it? Larger line and reservoir goes to the front 1/2. Obviously, there is air in the rear 1/2. Need to fix that problem next. If the system is completely dry it may not self bleed. Get a helper and try bleeding it. Don't dwell on the brake pressure warning switch, it is working as designed. Fluid in the front 1/2 and none in the rear will turn the lamp on, differential pressure.

Click to expand...

I believe it's the right master and booster. It's a 3/4 ton truck. Although I had to take the word of the local NAPA.

Another update:

Got the prop valve centered and went to the back. Rear rubber like was the issue. Looks okay on the outside, but completely blocked internally. I figured if the rear rubber like was bad, it wouldn't be long before until the front ones were bad. I don't see how gm decided it was a good idea to place the brake line/rubber hose connection inside the frame. I fought for hours on the passenger side. Ran out of time.
Will do driver side, bleed whole system and adjust rear drums.

For rear drums. I don't need to take my axle out to adjust do it?

There should be opening slots in backing plate to adjust star wheel through. Not every axle has an access but most do.

You should confirm that the break pedal itself is fully returning to the top of its stroke if you haven't. If something has been modified with it or possibly even the wrong length pushrod installed, the master cylinder will not function as it should.

So I’m late to the party I see with this post but I wanted to give my two cents anyways…first I’d like to say great job on the fronts they look way better. Good decision with the booster but I’m not sure I saw if you replaced the master also? That’s probably the first thing I would have replaced. The next thing is I’m betting the rear lines have collapsed plus you have frozen wheel cylinders. I’ve run into that issue multiple times where I can’t bleed them because the cylinders are rusted frozen in place. I ran into that issue recently with mine but the bleeder valve snapped off in the process so I’m going to be replacing everything for the rears including the e-brake lines and if I can’t resurface the drums I’ll get new ones as well. You definitely should get a helper to bleed everything it makes the process so much easier