How To Fix a Spongy Brake Pedal Chevy Truck

[EricTheCarGuy]

I don’t want to leave you hanging with this one. I’m likely going to try and replace the master and booster with the parts recommended to see if that actually works, but I can say for sure that just replacing the master cylinder didn’t do much for this truck.

I believe they designed the truck that way to make for safer braking during towing.

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[Mechmann]

Hi Eric,
(Please excuse this reply if you have already seen it as I may have placed it in the wrong category.)

Your Dad’s truck has a combination brake valve (a proportioning valve,a rear wheel pressure hold-off metering valve, and a failure mode pressure switch). You did not mention it during the bleeding process at the rear wheels, but the little rubber button on the end of the unit (metering valve) must be held in using a makeshift tool or by having an assistant hold it in. Otherwise, it is my understanding that the rear brakes will not bleed properly. Hope this helps.

Great step-by-step “how-to” video.

[EricTheCarGuy]

I’m aware of the tool and it’s purpose. I don’t believe it applies here because I believe the prop valve is housed in the ABS unit on these set ups which would mean that would not apply to this set up.

[Mechmann]

I’ve attached a schematic of your dad’s rear anti-lock brake system. The anti-lock mechanism as shown in [attachment=9167]Rear Anti-Lock Schematic (manufactured by Kelsey-Hayes…now TRW) only includes the Isolation/Dump Valve and an Accumulator (not shown) to store the released fluid. The metering valve, brake light switch valve, and proportioning valve are still contained int he Combination Valve. The problem seems to stem from the brake light switch valve moving from the pressure imbalance during bleeding and thus not permitting full flow to the brake caliper being bled. Two methods I have seen work. One is to hold the rubber button in while bleeding as recommended by the factory or alternatively purchase a special pin that takes the place of the safety switch while bleeding that keeps the valve from shifting. (Ref: https://www.performanceonline.com/Combination-Proportioning-Valve-Bleed-Tool/ )

Good luck with the bleed. This has been a problem for many other vehicles using this combination valve.

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[EricTheCarGuy]

Thanks for the info. This is like the old braking systems without ABS. I’m not sure if that’s the set up the truck has, but I’ll check it when I get a chance. If it has the set up you say, I’ll see if I can bleed it as you recommend. Thanks again.

[Author]

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