Started by johnfrederick, April 20, 2024, 04:49 PM

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Greetings all. I am trying to bleed brakes on my 84 Chieftain P-30 chassis, 23 RG. After trying everything I ended up having to remove the rear tires and hubs to get to the bleeders. I finally was able to bleed the rear brakes the old way with a friend pumping the brakes.

On the front I had previously installed new calipers and bleed then without any trouble. However, this time I couldn't get any fluid to come out of the bleeders. Then I discovered that when the brake pedal was pushed fluid blew out of the top of the open master cylinder. 

I can't figure out what is going on. Thanks in advance for any suggestions.

Thanks, John

PS, I've owned two of the pump kits for bleeding brakes. Both broke before the jobs were finished.



Take a look at what DaveVA78Chieftain posted way back when in the initial post of this thread..

The Chevrolet/GMC Combination Valve..It's job is to shut off 1/2 half of your brake system..front or rear..if you have a major failure..broken line..blown wheel cylinder..etc..

But it can create the problem you are having when bleeding brakes..When you open the bleeder valve, it causes this combination valve to shift towards the side you are bleeding..creates a problem in other words..

Solution is "centering" the valve so brake fluid flows normally..If it shifts to one side, that 1 side can't operate properly..

From what I understand, when you first turn the ignition on, it test the light on the dash "Brakes" then is suppose to go out after starting the engine..Is this light still on when engine is running??

If it is then that Combination Valve is activated (shifted) and that is the problem..

The best mechanic is the one that can make it run with the least amount of parts!


Thanks, Mike. This project started when I pressed on the brakes and the pedal went almost to the floor and the light came on. It recovered quickly, the light went out and has not happened again since.

Being concerned I started to check things out. I began by bleeding the brakes.

So, here is where I am at. The back brakes are bleed starting with the passenger side then driver's side. After bleeding the back, I started the engine to turn the steering wheel to place the front passenger side in position to bleed. No fluid would come out. Air? or fluid? shot out of the master cylinder. As they have easily bled in the past, I am hoping that it is something simple.

Thankfully, I have plenty of pedal. My plan is to put the rear back together and move on. If I end up not having front brakes it will let me know.

I am not sure if this classic has hydro boost or not. Anyone happen to know?

Appreciate any feedback!

Thanks, John


Eyez Open

GM brakes are extremely simple. Loosen the brake line attached to the master cylinder, pressurize the brake. If no fluid leaks/sprays out you have a bad master.

Gotta ask..was that brake fluid squirting out of the master or airbubles?

If fluid does flow next stop the brake line..drum/caliper...etc etc.

Master cylinder

Brake line

Portional valving........nasty little thing. Should be laying on top of cross member oil pan right above it. Could also be like a Easter egg hunt trying to find it.

Brake line


The above are all potential speed bumps, check one at a time starting with the master cylinder.


Eyes, thanks for responding. Master cylinder is new so for now I'm thinking that is not the problem.

The master cylinder was completely full when bleeding began on the front disc brakes. Fluid would not flow thru the bleeder.  Fluid or air back fed thru the master cylinder when the pedal was pressed. This back feeding splashed fluid out of the reservoir.... I'm not sure whether air or fluid was back fed. 

The engine was started off and on during this process to position front tires. I am thinking that may be part of the problem as the valves you mentioned did their thing.

What think you ?

Thanks, again!



Quote from: johnfrederick on April 21, 2024, 05:44 PMFluid would not flow thru the bleeder. 

If this valve is shifted to the rear brakes or the front, this is exactly what you will get...No flow from the bleeder..because this valve blocks the flow..

Get a DVOM (Digital Volt Ohm Meter) and see if this electrical contact is grounded or open (red arrow)..If it is grounded, then the valve has shifted..This is the ground for the light on the dash..

The problem with these valves is they are full of rusted mud brake fluid..causing them to jam up and not return to the neutral position..
The best mechanic is the one that can make it run with the least amount of parts!

Eyez Open

New cylinder that makes it much easier...ok how did you bleed the master cylinder would be the first question. Asking for a friend of course.

Eyez Open

If you've effectively bled the master next step is the proportional valve. Simply pull the exit line pressurize the system and verify. Just follow the links down.

It's not at all uncommon for these old p32 to have soft compliant rubber brake lines. That are rotten/deterioration in the core. Meaning the lines are shot and plugging up solid.