2013 - Master Cyl & Proportioning Valve Question

Today, I installed the SS brake lines purchased from OCTaco. The install was very straight forward, no issues there. However, when it came to bleeding the system, I hit a small speedbump...

Got everything setup to take on the rear-passenger drum, friend presses brake, I open the bleeder valve, some fluid and air comes out, then nothing. Tried again, but this time nothing came out and he said the pedal didn't move at all once I opened the valve. Tried it a couple more times, but still nothing. Moved over to the rear-driver drum, the pedal still won't depress, and fluid won't come out.

We weren't too sure what to think of this, so moved to the front, and both discs bled just fine, then back to the drums. After doing some research we guessed it had to be something with the Master Cylinder/proportioning valve (which I believe is basically built-in to the M/C on 2nd Gens).

Our first thought was to turn the truck on, step on the brakes a few times and try to let the valve reset itself. However, I was pretty scared about how finicky the ABS system is and nixed that one. We disconnected the two rear lines from the M/C (one at a time) and force fed brake fluid through them until it was flowing out the respective bleeder valve. Now that my lines were 99% air free, tied everything up and took it for a drive. Didn't have any issues, brakes felt good, no CEL, etc.

After the drive, I wanted to flush the lines again, but the same thing happened when I opened the rear-passenger drum. Fluid came out for about 1 second, then just stopped. Rear-driver wouldn't bleed either. What is causing this? I've heard of some pressure differential switches near the M/C in installations with disc and drums. Would this lock out both of my independent rear feeds from the M/C if the valve/switch thinks there's a major leak?

 

Just finished the SS brake line install and full fluid flush.

Key has to be on to do the flush and the ABS will beep and stuff, Its all good.

 

The key has to be in the on position while bleeding? Or did i read that wrong..

 

Yes it wouldn't bleed without the key in and on acc.

Key out: first pedal pump fluid bleeds, second and so on pedal pump nothing comes out. At first I thought the new brake lines were clogged, put the key in and bingo.

 

Ah..bet that's just required for the pedal method. I use a hand vacuum pump today and it seemed to work fine..other than the pedal going down a bit further to brake it seems to have worked.

 

I use ATO DOT 4 blue racing brake fluid, it comes in blue and yellow. The different color fluids make pedal pump flushing a snap. Plus you can go longer between flushing, I have been using it for years in my other car.

 

Interesting, that's kind of what I was thinking, but afraid to try it. In two different posts, you said the truck was "on" and the other said "acc". Which one was it? Just trying to see if "on" triggers something else with the ABS system, where "acc" doesn't. Also seems like you're the first one to mention this as a necessary step to bleeding the rear brakes...

Can anyone comment on how our brake system works exactly, and what the proportioning valve is doing with the truck off/on?

 

Unless stuff has changed, I think the proportioning valve is "dumb", meaning it doesn't have a computer controlling it. It only controls the bias between front brakes and rear brakes. Because disc brakes/front require more percentage of the braking power vs. drum brakes/rear, this is all it does. It keeps the rear from locking up first before the front brakes have a chance to do their job.

I think the key on thing has more to do with the ABS pump and module. All the ABS is a smart pump. Since you introduced air into the brake system when you replaced your brake lines, there is likely air in your ABS. Having the key on is going to help you purge the air through your pump.

If you were just flushing your brake fluid with fresh stuff, the way you were bleeding the brakes would have been fine. You would have had fluid flowing each time you opened the bleeder valve at the rear.

 

Sorry about that, I meant key in and on in acc mode (engine not running). Don't know about being the first to mention this but completed the flush on Sat morning and have been driving the truck the rest of the weekend with no problems.

 

Either way, air is a fluid which should be pushed through just the same as brake fluid, wouldn't it? From what I've read, once air gets introduced into the ABS system and/or M/C, that's when there's major problems --> tow truck to dealer. So I don't think that happened, but not 100%.

The second time I tried to bleed (after taking my successful test drive), the rears were cutoff after a minor outlet of fluid. And this was after manually filling the lines with brake fluid, extremely small amount of air in lines.

From what Matt is saying, it sounds like the truck would have to be in "acc" whether bleeding after a line change, or to perform a full flush.

Guess I should have been more clear too, I meant to ask "acc" or "on" without the engine running. Sounds like "acc" is enough to activate the ABS pump, or whatever is required to allow rears to bleed. I'm just still confused why the ABS pump would affect the rear drums, and not the front discs. I thought the only device to "control" disc vs. drum pressure was the proportioning valve. I don't remember having this issue on a vehicle with all around discs.

 

This is the first I've heard of this. When bleeding my brakes we used the pedal method, key was not in the ignition and had no issues. Perhaps this only occurs with the newer model trucks, mine is an 05?

 

Thanks for the input, that's basically what I'm trying to help figure out for anyone else. Here's what my setup looks like at the M/C (2013 DC Offroad)

 

Is your brake light on after starting the truck?

 

Red "BRAKE" light on dash? Nope, I thought this only comes on when the parking brake is applied. Is it also a trouble indicator?
Amber "ABS" light on dash? Nope

There's nothing signaling to me that there's an issue with the truck. I'm driving it like everything is fine and nothing has concerned me yet.

 

That light will also come on if fluid is low I think

 

The yellow "ABS" light will come on when you first key up the truck as a system and bulb test. After engine is running it should only illuminate when there is an ABS problem.
The red "BRAKE" warning lamp comes on when the fluid level sensor indicates low fluid, the parking brake is on, or there is a problem with the base brake system. (Non-abs)

As for what the proportioning valve does, the answers given are correct in that it is a "dummy" valve that makes sure that even braking front and rear is present. The front brakes require more fluid to apply that the rear brakes do, when you have a disc/drum brake system. The proportioning valve makes sure that when you apply the brake pedal more fluid is applied to the disc brake to "even out" the braking force.