Why cant I bleed my rear brakes?

[slow-1]

Originally Posted by LONGHAIR

Disc front, drum rear?
Sounds like the proportioning valve is popped. It is a safety system that "cuts off" one end or the other if a leak develops. It "balances" between the front and rear until one end loses pressure.
Bleeding the front first, especially with the problem in the rear, made it worse.
You need to get it re-centered first, then the rear will bleed.

How do you "recenter" the prop valve?

[Daddy Brim]

Originally Posted by slow-1

How do you "recenter" the prop valve?

Stomp on the brake peddle like your going 100 mph and need to stop now. You may need to do this several times. You may also hear a pop or a click when it resets.

[slow-1]

Originally Posted by cdowns

sooo many threads about this.. doesnt anybody read a service manual ever?? the CORRECT procedure is in there in detail

clamp the pin on the combination valve

Clamping the pin gets the gel out of the bleeders?

[Mister-B]

Originally Posted by slow-1

Clamping the pin gets the gel out of the bleeders?

Lol. Don't mind him.
Posted via Mobile Device

[slow-1]

So it looks like there ar LOTS of reasons I cant bleed my rear brakes.

1 - Gummed up bleeder passages

2 - Prop valve tripped (more chicken dinners due).

3 - Another brake line rusted through enough to suck in air when I use my handy vacuum bleeder.

4 - Who knows

I also poked a hole in my radiator while it was out of the truck during my engine rebuild with the cherry picker, then the replacement I bought doesn't fit...this truck is really challenging my sanity

[LONGHAIR]

Originally Posted by slow-1

How do you "recenter" the prop valve?

Daddy brim's method works most times. It is not as much about pressure as it is speed. Stomp it quickly, it may take a few times.
If that does not get it, (you are going to hate this) open one of the front bleeders and push the pedal too the floor. When it gets close, you will feel some resistance, push past that and you will feel/hear the click. Tighten the bleeder. Now start the bleeding process again, from the right rear, working toward the closest.

[slow-1]

Originally Posted by LONGHAIR

Daddy brim's method works most times. It is not as much about pressure as it is speed. Stomp it quickly, it may take a few times.
If that does not get it, (you are going to hate this) open one of the front bleeders and push the pedal too the floor. When it gets close, you will feel some resistance, push past that and you will feel/hear the click. Tighten the bleeder. Now start the bleeding process again, from the right rear, working toward the closest.

I can tell you that this method does NOT work if you still have a leaking brake line.

I am learning more about 40 year old brakes than I ever wanted to know.

I did take the light switch out of the prop valve and was able to easily center the internal safety valve using an ice pick. Of course it tripped immediatly when I pushed the pedal because of the yet undiscovered leak.

[LONGHAIR]

Originally Posted by slow-1

I can tell you that this method does NOT work if you still have a leaking brake line.

Well you can bleed the system with a leaking line anyway

Seriously though, it is much better to find it while bleeding than when you need the truck to stop.

[Lee H]

There is a tool that keeps the proportioning valve pin set in the correct position while bleeding the brakes. You can bleed without it but not as easy. Also, the correct bleeding procedure, according to the GM service manual, is closest to farthest, unlike most cars. I know lots of guys will say nay to the tool and the closest to farthest but it is my humble opinion, that leads to a lot of these "they can't I bleed my brakes" threads.

[Mister-B]

Originally Posted by Lee H

There is a tool that keeps the proportioning valve pin set in the correct position while bleeding the brakes. You can bleed without it but not as easy. Also, the correct bleeding procedure, according to the GM service manual, is closest to farthest, unlike most cars. I know lots of guys will say nay to the tool and the closest to farthest but it is my humble opinion, that leads to a lot of these "they can't I bleed my brakes" threads.

That's an interesting take. What side on the rear do you consider farthest away? I think most people would assume the right rear (because it is opposite the MC), but on these trucks it may very well be the left rear, because the one line goes front to back, and then it has to travel further to get to the left side
Posted via Mobile Device

[slow-1]

Having recently been laying under my truck studying the brake lines, I would agree that the left rear is farthest from the MC...but only by a few inches. The one line to the back (that runs up the right frame rail) splits at the end of the hose running from the end of the hard line to the top of the differential. The bracket is mounted on top of the rear end housing but off-center to the right causing the line to the left rear to be an inch or two longer than the one to the right.

[slow-1]

Here is how I recentered the safety valve in my proportioning valve.

That is the prop valve under the master cylinder.



Closer view.



With electric connection removed from the switch that turns on the brake light on dash when safety valve pops.



Switch removed.

[slow-1]

Looking down into the hole the switch came out of.



Using a pick to slide the valve back to the center...my leak was in the rear so the valve moved to the rear to block fluid loss.



Centered.

[slow-1]

By the way, with no leaks in any brake lines, I can bleed the brakes in about 5 minutes, front and rear, with my vacuum bleeder...very easy.

[special-K]

The rear brakes are fed through the same line,so left or right is no different as far as farthest

[slow-1]

Originally Posted by special-K

The rear brakes are fed through the same line,so left or right is no different as far as farthest

On mine the split is off center of the truck to the right so the line from the split to the left rear is 2-3 inches longer....probably differs depending on when and where the truck was built.