Brake system help!

[Gibson]

That I'm not sure on. I've heard of people trying to "upgrade" to the NBS master cylinder or brake booster, and just making things worse.

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There's a lot of truth in that.
The NBS masters are designed for a disc/disc system, and generally speaking, the piston that feeds the rears does not move the volume of fluid that is necessary to expand-out the wheel cylinders without excessive pedal travel.
Yeah,, bleeding can be a real pita,, sometimes you end-up just moving the air back-an-forth with the brake pedal, using a lot of fluid for little results.
On thing, don't ever shake a can of brake fluid before you pour it in the master to start bleeding,, you'll just fill the fluid with a million tiny bubbles,, and those bubbles will eventually all rise-up and blend together into one big bubble.
A good pressure bleeder is your friend.

 

[RawbDidIt]

There's a lot of truth in that.
The NBS masters are designed for a disc/disc system, and generally speaking, the piston that feeds the rears does not move the volume of fluid that is necessary to expand-out the wheel cylinders without excessive pedal travel.
Yeah,, bleeding can be a real pita,, sometimes you end-up just moving the air back-an-forth with the brake pedal, using a lot of fluid for little results.
On thing, don't ever shake a can of brake fluid before you pour it in the master to start bleeding,, you'll just fill the fluid with a million tiny bubbles,, and those bubbles will eventually all rise-up and blend together into one big bubble.
A good pressure bleeder is your friend.

With stubborn bubbles, I've had good luck with sealing up the system and pumping the brakes in between bleeding rounds. Especially with old fluid or if you make the mistake of shaking the fluid before pouring, As mentioned. I make a habit of never shaking any automotive fluid before pouring it. I treat it like beer: tilt upside down for 30 seconds before pouring.

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[Jeremy Lueders]

In my experience and from everything you have already done, I would consider that the master cylinder is bad. I was working on an old truck of mine and did all the same things you did just to find that when the old line blew-out it took the master cylinder with it. I didn't know it until I thought I had it bleed and it would go to the floor. Just my 2 cents.

 

[JJ Burgess]

In my experience and from everything you have already done, I would consider that the master cylinder is bad. I was working on an old truck of mine and did all the same things you did just to find that when the old line blew-out it took the master cylinder with it. I didn't know it until I thought I had it bleed and it would go to the floor. Just my 2 cents.

Thank you. I just ordered another one. Just going with the standard OBS one. It’ll be here tomorrow. Hopefully that does the trick.

 

[cgreppi]

I couldn't find a part # for the combo valve anywhere, so I took my old one off, and compared it to others I looked up on a Google image search. I ordered this one from Amazon:
ACDelco 172-2069 GM Original Equipment Brake Combination Valve

It was mounted backwards on the bracket that didn't fit anyway, so I took it off the bracket and put it on my original bracket. I also deleted the RWAL system, and bench bled the new master cylinder. I have a firm pedal now, but I don't think the brakes have just a lot of power, I just don't know what they should feel like cause they were horrible when i got the truck.

I hate to say it but that
ACDelco 172-2069 Combination Valve for Chevy S-10 & S-15 Blazers

You want 172-2184 AC Delco, 1988-1990 Chevrolet C1500 Brake Proportioning Valve
you can purchase it here:
https://piratejack.net/172-2184-ac-delco-1988-1990-chevrolet-c1500-brake-proportioning-valve/

Also buy Proportioning Valve Brake Bleeder Helper $5.00

 

[Schurkey]

Also buy Proportioning Valve Brake Bleeder Helper $5.00

https://piratejack.net/proportioning-valve-brake-bleeder-helper/
Wow. The boneheads selling this are calling it a "Proportioning Valve" brake bleeder helper.

The thing has NOTHING to do with the proportioning valve. It screws into the Combination valve in place of the safety switch. I suppose that makes it a "Safety Switch Brake Bleeder Helper".

And the genuinely frightening thing, is that all the GM combo valves I've had experience with, use a self-centering safety switch. I've seen some older Fords that had to be reset manually--which would make this tool useful. But if the safety switch is self-centering, this tool is useless. The whole point of the tool--literally, as in the little pointy stem at the bottom--is to keep the safety switch centered.

NOW I have to wonder if this aftermarket Combo valve has a GM-style self-centering safety switch, or if they "forgot" to include that feature. Maybe GM quit using self-centering safety switches. Could be. I doubt it, but I haven't worked on every application.




For the record, in a disc/drum system where the hydraulic system is split front/rear, the Proportioning valve affects the REAR WHEELS ONLY. It has nothing to do with front brake pressure. The Hold-Off (Metering) valve affects the FRONT WHEELS ONLY. It has nothing to do with rear brake pressure. The safety switch is between the front and rear hydraulic systems. It is pressurized on both sides, if there's no difference in pressure, the valve doesn't move and the dash light stays off. If one side loses pressure due to some hydraulic failure anywhere in the system, the valve is pushed to the side by whichever hydraulic system is building higher pressure. Then the light on the dash comes on to tell the driver what he should have already figured out--half of his brakes aren't working.

 

[JJ Burgess]

Verify you have adequate fluid in the master cylinder.

Jack up the rear until the master cylinder is AT LEAST level, preferably down in front. If you can't jack the rear high enough, you'll have to pull the master off of the booster and push the front down--then move the piston with a blunt object (wood dowel or Philips screwdriver). DO NOT DISCONNECT THE BRAKE TUBING.

Open the master cylinder lid--or not. Your choice. If i had a helper, or I could see it from the driver's seat, I'd want to see bubbles. If I didn't have a helper, I wouldn't bother.

Tap brake pedal MORE than what's required to take up all the slack in the linkage between pedal and booster, and between booster and cylinder. You have to move the primary and secondary pistons at least a little. Going farther than that is pointless, because the piston seal covers the bypass port. It may be that the air bubble ahead of the primary piston will have to bleed out into the reservoir before the secondary piston will start to move. Then the secondary air bubble will blow into the reservoir. When there's no more air, you'll see two little "geysers" of fluid (not air) pushing up from the bypass ports when the pedal is tapped. You'll see these same "geysers" with the vehicle level and the master tilted upward--but there could still be air ahead of the ports. That's why you have to tilt the master DOWN in front to release the air.

Tap the brake pedal multiple times. If you're watching the reservoir, you'll see when there's no more air. If not, you just have to tap and hope.







There was a time--thousands of years ago, when Dinosaurs roamed the Earth--that master cylinders had their own bleeder screws, and then all this crap wasn't needed. 'Course, bleeder screws cost money, as does drilling and machining the casting to accept them, so bleeder screws on the master disappeared a long time ago.

You've never said whether you bench-bled the master cylinders before installing them. SOMETIMES bench bleeding will eliminate this problem--depends on how carefully you bench-bleed, AND how careful you are to seal the ports during the installation, AND how fast you can connect the brake tubing once the thing is bolted on. Tilting the master cylinder like I've told you, because bench-bleeding wasn't enough, is really common.

So I tried this yesterday. No luck. I had the back up very very high. Had a buddy watch as I tapped the pedal. No bubbles. Just fluid movement like a geyser. So I’m going with a bad master cylinder. I’m going to change it and see what happens. I’ll keep ya posted.

 

[Schurkey]

I had the back up very very high. Had a buddy watch as I tapped the pedal. No bubbles. Just fluid movement like a geyser.

Doesn't matter how high the back end is. What matters is the angle of the master cylinder. As long as it was down in front, and you had TWO geysers, (one in each section of the reservoir) but no bubbles, there's no air trapped in the master, and I was wrong.

If the master still had the front tipped up, you need to do this again.

 

[JJ Burgess]

The

Doesn't matter how high the back end is. What matters is the angle of the master cylinder. As long as it was down in front, and you had TWO geysers, (one in each section of the reservoir) but no bubbles, there's no air trapped in the master, and I was wrong.

If the master still had the front tipped up, you need to do this again.

front was definitely tipped forward. I’m gonna try another master and see what happens.