89 C1500 rear brake issue

[rs74]

I dropped my truck off at a shop a while back since I have not had time to get to it. One of the things it needed was the rear brake line that runs along the frame rail behind the fuel tank replaced due to it starting to leak. I went in to check on it today since I had not heard anything for two weeks and he told me that he couldn't get any fluid to the rear when bleeding. He told me that he could get fluid from the master cylinder to the abs unit but nothing to the rear. I am planning on bringing it home either tomorrow or sunday to try and see what is going on myself. Any ideas on what would cause a no fluid/pressure to the rear? I have read something about maybe needing a specific tool for the distribution block to keep a valve centered?

This is my late father n laws truck and has sat for two years. I ended up with it so my wife could have one of her dads trucks. Need to get the brakes fixed so I can see about having cab corners put on. I will say that I had went about a month ago to pick it up at the same shop after putting a fuel pump in and that is when I noticed the brake leak. I drove it up to the gas station to fill it up after getting it and the pedal seemed pretty hard like the brakes just wasn't working. I noticed the leak at the gas station and drove it back to the shop. The pedal seemed pretty hard the whole time but never had good brakes. Even with brake fluid leaking out of the line it never even acted like it wanted to go to the floor like you would think it would.

[Palf70Step]

I have heard of that issue before, but I cannot remember what the solution was. I didn't think the rear ABS didn't show up till the 92 year ( or maybe it was just RAWL until 92), but I still have a lot to learn on things. If it is RWAL ABS, then there is a special bleed method, but I don't have my service manual any more to look it up.

Found this, seems like it is right
https://itstillruns.com/bleed-abs-br...-12157633.html

[rs74]

I will have to wait until next week to dig into it. I dig go pick up the truck and drive it home today. Before going any further I should state that when I picked up the truck from the family farm the rear brakes were locked up. It seemed like the brakes were seized up from sitting and a bunch of brake dust in the drum. Took the drums off and cleaned it all up and put back together to get it to roll.

Alright, back to driving it home today. It didn't feel like I had any front brakes grabbing at all. It felt like it was only rears and not very good at that. Brake light was on also. I don't know if the valve in the isolation? valve could have moved when the brake line started leaking or maybe something happened before I picked the truck up and wasn't told about it. I will just have to dig into it and see what I can come up with.

[Second Series]

I have an '88GMC and a '90chevy, they both have the RWAL system. You could check for fault codes, you can also test the Isolation/Dump valve with an ohm meter. In the attached picture, you can see the Isolation/Dump valve on the left. Combination valve on the right with the brake control module attached to it. Here is some info that I found in a couple other posts about the subject:

There are three brightness levels of the dash brake indicator light on RWAL equipped trucks.

Brightest – park brake
Medium – Combination valve (aka metering & proportioning valve)
Dim – RWAL System

Rear Wheel Anti-Lock Brakes May Be the Problem
This is a known issue on Chevy C1500 pickup trucks equipped with rear-wheel anti-lock (RWAL) brake systems. There is a small black module near the master cylinder that serves as the brain for this system, and there are ways to check for diagnostic trouble codes. Once a code is revealed, diagnosis can be fairly simple.
Checking the Diagnostic Trouble Codes (DTCs)
Diagnostic trouble codes (DTC) can be displayed by jumping terminal A to terminal H on the Data Link Connector and observing the flashing of the BRAKE warning light on the dashboard. This test should only be performed when the BRAKE warning lamp is glowing.
The terminals must be jumped for about 20 seconds before the code will begin to flash. Count the number of short flashes, starting from the long flash (include the long flash as a part of the count). Sometimes the first count sequence will be short. However, subsequent flashes will be accurate. If there is more than one failure, only the first recognized code will be retained and flashed.
Diagnostic Trouble Code Notes
• DTCs 6, 9, and 10 will only turn the BRAKE lamp on while the fault exists. These are soft codes faults and can only be read with the Tech 1 scan tool.
• Do not jump terminal A to terminal H if the BRAKE dashboard lamp is not on. Jumping the terminals while the BRAKE lamp is off will set a false DTC 9. This will turn the BRAKE lamp on and disable the anti-lock system.
Interpreting the Brake Lamp Code Flashes
• 1, 11, and 12: ECU (engine control unit) Malfunction
• 2: Open Isolation Valve or Malfunctioning ECU
• 3: Open Dump Valve or ECU Malfunction
• 4: Grounded Anti-lock Valve Reset Switch
• 5: Excessive Actuations of the Dump Valve
• 6: Erratic Speed Signal
• 7: Shorted Isolation Valve or Faulty ECU
• 8: Shorted Dump Valve or Faulty ECU
• 9: Open Circuit to Speed Signal
• 10: Brake Lamp Switch Circuit
• 13, 14, and 15: ECU Malfunction


The combination valve is out by the master cylinder and has only one wire going to it.

The isolation and dump valve are in the same part. It’s located out by the master cylinder.

The isolation/dump has 4 wires going to it and has two solenoids on it

If you have a good digital volt ohm meter here are some tests you can do.

Key off, isolation dump valve unplugged, probing the wire connector that is on the Isolation dump valve.
Terminal A to terminal D - should be 3 to 6 ohms.
Terminal A to the valve housing – should be greater than 50,000 ohms.
If the resistance is to low on either of these tests, the valve is shorted. – replace it.

Isolation dump valve disconnected, Key on, ALDL Terminal H connected to terminal A – test voltage between terminal A and D of the harness that goes to the isolation dump valve. You should have one volt or more. If it fails this test it points to the RWAL module.

A = Green wire
B = White wire
C = Blue wire
D = Black wire

Here is the way the wires are positioned on the RWAL module:
D = Black wire
C = Blue wire
B = White wire
A = Green wire

If it passes all tests, clear the code and see if the same code comes back.
On early S trucks, the ECM B fuse powers the RWAL.
Clearing codes
You can clear the code by removing the power to the RWAL module for 10 seconds. Multiple circuits send power to the RWAL module and the fuses may be labeled STOP/hazard and BRAKE. Removing a fuse labeled RWAL may not clear the code. GM information on which fuse/fuses to pull is not very reliable.

[ChevyTech]

RWAL was standard equipment on all C/K trucks beginning in 1988.

Here are two threads on RWAL from the FAQs thread at the top of this board.
http://67-72chevytrucks.com/vboard/s...d.php?t=302690
http://67-72chevytrucks.com/vboard/s...d.php?t=399153

Here is the FAQ thread which has several topics covered for these trucks.
http://www.67-72chevytrucks.com/vboa...d.php?t=445487

My best guess for the problem with the truck rs74 posted about is that the plunger in the one wire combination valve is not centered and most likely stuck at the end of its travel.

[ChevyTech]

Here is a thread for the exact problem rs74 is having.
https://www.chevytalk.org/fusionbb/s...hp?tid/166852/

To get fluid to flow to the rear brakes again try this:

Have someone hold pressure on the brake pedal

Open a front brake bleeder. This should move the plunger in the combination valve if it is not stuck bad.

Close the front brake bleeder.

Let the pressure off the brake pedal and do not push it any more.

Open a rear bleeder and watch it for several minutes and see if fluid will start coming out the bleeder on its own. It should if the combination valve was the problem and it moved.

Here is a good thread from another board I post on that covers RWAL.
https://www.chevytalk.org/fusionbb/s...php?tid/262169


CT

[speedygonzales]

You could also consider making lines that connect the MC directly to the rear lines and bypass the ABS either temporarily until you figure out the ABS, or permanently.

That would also make it easy to troubleshoot any problems as it eliminates the ABS equipment from the equation and leaves you with MC, front calipers and rear wheel cylinders only to mess with.

[hatzie]

I'm not a fan of the RWAL systems. It's fairly easy to bypass the ABS motor permanently with a single chunk of hard-line.

You'll still need to get the Combination Valve shuttle to centre so it's not blocking off the rear brake circuit.

Once the valve is centred you'll need to remove the warning lamp switch and screw in the tool to lock it in place.


Then you can bleed the brakes. I'd question the abilities of your mechanic since he can't handle something this basic.

This is a cutaway view of a generic combination valve with the shuttle centred and the lock tool screwed into the switch location.


If the valve is locked up so it's not moving when you open the front brake bleeders and pump the pedal you can remove the rear brake line and push the valve forward with a probe til you can see the groove under the switch plunger hole. Then screw in the tool and bleed the brakes out.

I use a weed sprayer pressure pot bleeder. Motive Products bleeders are so cheap now it's not worth building your own.