Gumby22

I see this topic a lot. People struggle with bleeding brakes on these trucks (understandably so) and I find I'm often repeating the same answers over and over again.

Therefore, I thought it would be helpful (for myself and others) to have a thread specific to these trucks since there's no sticky/help section on this board (GMT400 specifically).

PLEASE NOTE: The information written in this post may be contradictory to some of my previous posts - all I can say is I do the best I can to answer questions thoroughly and provide factual information at any given time. Occasionally I get said facts mixed up and/or forget to include necessary information, so if any of my previous posts have led anyone astray, I apologize, and that's yet another reason to have a post I can continually reference in the future.

This information applies to any GM truck that uses a rectangular style master cylinder reservoir and has a combination metering/proportioning valve. Most GM trucks switched to an oval shaped reservoir beginning in about 2000 but some of the C3500 trucks used them up to 2002.So, why is it such a hassle?



Cutaway view of combination valve.

The combination valve on these trucks utilizes a pintle valve to act as the metering portion of the valve - it restricts fluid flow/pressure to the front brakes until adequate pressure is applied to the rear brakes in order to provide equal hydraulic pressure between both circuits. This restriction oftentimes causes air to get trapped in the valve and/or individual hydraulic circuits because it's difficult to build adequate pressure and flow to expel the air from the system using manual (pedal pumping) bleeding procedure.

To prevent this restriction during bleeding, a clip tool is placed on the end of the metering rod to hold it inward, allowing fluid and air to flow freely around the valve and out of the hydraulic circuits. The OTC tool# is 7853 but there are many other brands available.

The picture below shows a typical combination valve for these trucks, though exact design will vary depending on year.

As you can see, there's more than 1 potential location for the metering rod. The factory will typically orient the valve so the rod is facing up or forward, but depending on the year, model and options, and any previous repair work, that's not always the case.

To identify the rod, have a helper pump the brake pedal while observing the valve - it's usually under a black rubber cap and will extrude when the pedal is pressed. That is the rod that needs to be clipped down during bleeding. If the black cap is still present, remove it to install the clip tool.



Depending on your access to tools and materials, and the accessibility of the valve itself, there are a number of different ways to secure the rod in lieu of a clip tool. 2 possibilities are a C clamp large enough to span the valve or a dime secured with a zip tie wrapped around the valve.

Once this rod is secured, standard brake bleeding procedures can commence. The traditionally appropriate sequence to follow is right rear, left rear, right front, left front.

With manual bleeding (pedal pumping) you will want to follow the standard sequence, making sure to frequently check and top off the brake fluid as the rear portion of the MC reservoir will quickly drain.

With pressure bleeding, you can just about use any sequence you want as long as you're sure all the air is bled out. My personal method is to count the number of seconds until I see new, clear fluid coming out of the line, and then I count to double that amount, starting back at zero if I see any air. If I reach the target number of seconds without seeing air, I consider that line bled.

So, what can cause air to remain trapped, even if the metering rod is restrained?

Keep in mind that any truck with an original factory set up is at least 20 years old. Even assuming there has been brake system service and the fluid has been flushed, the system is often contaminated prior to service and it's quite common for sludge and debris to build up inside the master cylinder, combination valve, lines/hoses, calipers and wheel cylinders.

This contamination is not always completely removed during service for varying reasons, and often it creates restrictions within the system, so here's some tips on what can be done:

1) To prevent more contamination from entering the system, remove the MC reservoir and thoroughly clean it with hot, soapy water. While it's out, wipe out the bores in the MC with shop towels and brake clean.

Removal will be easier if you warm the seals with a heat gun. The seals can be reused as long as they're not damaged - lightly applying oil or silicone grease will make reinstalling the reservoir easy.

2) Remove all the bleeder screws, disconnect the primary lines from the MC, and flush them out with brake cleaner and compressed air (regulated to 20 PSI or less for safety reasons). Be thorough and repeat 3 or 4 times.

You can also shut off 3 bleeders to isolate the individual circuits and focus the air flow on each circuit individually.

Any remaining brake cleaner will be flushed out during bleeding.

3) Adjust the drum brakes all the way out - this eliminates movement of the wheel cylinders during bleeding which, in combination with restrictions, can trap air. It also provides a more accurate brake pedal feel for determining if the lines are properly bled.

4) Once initial bleeding is accomplished and all the bleeder screws are closed, pump the brake pedal several times and then hold it - if the pedal slowly sinks to the floor, there's still air in the system. If using a pressure bleeder, make sure to remove it prior to pumping the pedal.

5) If the pedal is still spongy after several attempts, inspect for leaks and kinked lines or hoses. Repair as necessary. You can also try depressing the front calipers with a C clamp while it's mounted on the rotor. This can push air back up to the MC and out through the reservoir.

6) Perform an ABS bleeding procedure. Theoretically this shouldn't have to be done during normal brake system service, but what can often happen is contaminated brake fluid can work its way into/around the individual valves for each circuit (especially if the ABS is activated during driving), causing the valves to stick, allowing fluid flow when there shouldn't be, and causing air to get trapped. Typically ABS bleeding needs to be performed with a scan tool, but if you're knowledgeable enough and have access to the appropriate wiring diagram, it can be accomplished using jumper wires at the ABS control module. If you're going to do this, don't activate the individual valves for more than a few seconds at a time.

The correct method for bleeding ABS is to perform base brake bleeding, bleed the ABS hydraulic valves, then bleed the base brakes again to remove the air that was expelled from the ABS valve.

7) If the master cylinder has been replaced, make sure to properly bench bleed it before installing it on the truck. Once installed, the MC is angled slightly up, and air can get trapped at the very end of the piston bore, beyond the position of the secondary line port. This air cannot be expelled using manual or pressure bleeding.

BONUS TIP: Some older model trucks have wheel cylinders that are mounted either vertically or at an angle. These are typically medium and heavy duty trucks that have 4 wheel drum brakes and no combination valve, but if you encounter one, there’s a couple of additional steps when bleeding:

1) After initial bleeding is done, back off the shoes as far as possible (adjuster screw fully retracted).

2) Have a helper pump up the pedal, then, crack the bleeder loose as the pedal is released, then immediately close it again. Repeat 3-4 times to purge the remaining air.

The reason is because, with the wheel cylinders mounted in an other-than-horizontal position, air gets trapped above the bleeder. These additional steps will remove that air, otherwise it would remain trapped and cause a spongy pedal.

I have to call this the end for now, but I do intend to edit & add information in the future as I think of it and have time to do so.

Edited 11-29-22 to modify/correct wording and add image.

oilcanhenry

Good job, Gumby22!

Gumby22

Thank you.

hanky

Thank You Gumby, for the explanation and the right way to handle brake work.

It is scary how many vehicles are out there with brake work done by a person that has no idea of how it should be done.
Some only come here after they have a problem, and the ones that don't know they have a problem or created one are still out there.
Thanks for helping make our driving a little more safe.

Gumby22