are talking about the valve in the rear that attaches to the leaf spring? I believe it is supposed to allow more brake pressure on the rear brakes when a load is on the truck. That valve, I eliminated on mine. It seems to work fine without it.
As far as bleeding, I never could get the rears to bleed with the vacuum unit. And this is the only vehicle I ever had that problem with.
I have a friend who is a retired mechanic. He suggested that with a helper, open the bleeder on the rear, slowly push the brake pedal down, tighten bleeder and repeat. This got the air out for me.

 

The valve in the photos above is on the front frame rail.
In the event of a brake line leak, it closes off the leaking brake circuit (front or rear) and turns the brake warning light on.

My trucks both had Rear Wheel Antilock, which uses a RWAL Modulator valve (which is connected to the RWAL controller)

Older trucks used a mechanical load based valve, which is meant to prevent the rear wheels from locking up when there is no load/full load in the bed.

You could go without either rear one, just like you can drive a vehicle with ABS disabled.
Running without the front valve would be foolhardy at best.

 

I always bleed brakes that way. you don't need a helper, just a piece of hose and a small plastic bottle full of brake fluid.

Vacuum systems can be a hassle. With some modern vehicles you can't use them at all because of the ABS system design.

 

Robert, I was not talking about eliminating the valve in the picture. FWIW I think that is referred to as a proportioning valve.
My comment was made based on post 65, and I quote. "]I've been fighting my brakes for a while now. I removed the hydraulic solenoid valve (on the frame, in the rear) because I couldn't bleed the rear lines past that point."
While my truck is a 87 it may be different then the 89 that the op posted about. I just don't know.
That rear valve is not available anymore, so eliminating it was my only option.

 

That rear valve is a load-based proportioning valve, and as mentioned, serves to "reduce" the rear brakes on a light load.

It can be done away with, but the cost is that the rear tires MAY lock up earlier.

RwP

 

Lots of good advice, thanks. I wanted to swing by the pick-n-pull and see if I could find a hydraulic valve (ABS) to replace mine, but its been a busy week so I haven't made it yet. I will probably by-pass the valve if I can't find an original replacement. I know you can buy adjustable after-market units too. As for the leaky connection at the combo valve in front, I guess I'll look for a flaring tool so I can put on the original line nut and get rid of the adapter. Anyone ever borrowed one from AutoZone?

 

OK, the leak at the front (at the combination valve) was due to the adapter. RobertMc, I think you put me on it. It wasn't tubing, but the adapter itself was not sealing.
I replaced the adapter with a new one and it appears to be staying dry. It felt like it was seating better too. With the old adapter the threads seemed to get tight before it even tightened down on the sealing surface.

I also temporarily bypassed the hydraulic ABS valve in the rear of the truck. This will allow me to finish bleeding the brakes, while I look for a replacement valve. This is the valve that has a bleeder screw on it, which broke off.

 

Great work on this!! Want to help fix mine up next?! Haha

 

"Next" could be a long time my friend, lol.

 

Guess I knew this was coming; was about to bleed the brakes a couple weeks ago and noticed a trail of fluid and loose paint on the brake booster, under the master cylinder. I wasn't sure how old the master cylinder on the truck was, but apparently it was old enough to rupture a seal after it sat up for the last few years. My plan was to use the original master cylinder and booster if there were no leaks; just clean them up and re-paint them. But now that the master cylinder has failed, I decided to replace the booster as well as the master cylinder, since those were pretty much the only brake-related components that weren't replaced in this rebuild.

The toughest part of replacing the booster is getting to the four nuts under the dash and above the steering column. The easiest way I found to reach them was to use a half-inch drive ratchet with a deep 1/2 inch socket, a universal joint, and a long extension. With the knee bumper plate removed from under the steering column, you can reach in, threading your extension thru the wiring harness. I was able to get all four of them this way. A good light helps too.

If you ever see this kind of leak between your master cylinder and booster, you've got a ruptured seal. If the leak is bad enough, and goes on long enough, fluid could get into the booster as well. Brake fluid is corrosive, so you don't want that to happen. If you think the booster may have fluid in it, my advice is replace it.




The rebuilt master cylinder and booster I bought came already paired up, but you have to separate them so you can bench bleed the master cylinder prior to installation. I'll be doing that soon. The booster had no paint on it, just a clean, blasted surface, so I primed and painted it.




Here's the hole in the firewall where the booster mounts. There's a foam gasket that goes in between the booster and firewall but my new parts didn't include it. The old one looks workable so I'll re-use it. I did get a gasket for the area between the booster and the master cylinder, which strangely enough, my original set up didn't have.