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Gm Tech (Cam)
Gm Tech (Cam), GMC Mechanic
Category: GMC
Satisfied Customers: 29427
Experience:  GM Grand Master Technician 2007. 14 year experience.
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GMC Sierra 1500 HD Pickup: Brake pedal still very spongy after

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Brake pedal still very spongy after bleeding 20+ times. I recently purchased a 1998 GMC Sierra 1500 extended cab pickup. I had to replace the entire brake system; front calipers and rear slave cylinders; pads and shoes; all pipes and hoses and the master cylinder. The only things not replaced are the 4WAL module and the power booster. The brakes work, back and front, to where I can skid (guess the ABS isn't working), but I can't get rid of the spongy pedal.

Extra note; before replacing the entire system, I noticed that brake fluid would shoot out of the 4WAL module for the front brakes, but only spit out of the hole for the rear. I haven't checked this since replacing everything.

I'm desperate. Winter is here, I really need the vehicle and we don't have any cash left to pay a mechanic...business is very slow...sob sob.
Hello, I'm Cam. Please include all the details about your issue so I can accurately advise you. I cannot see your vehicle so please be detailed.

There is air in the abs valve assembly. This makes regular bleed useless to get air out. To get the air out you can try first finding a place where there is gravel, ice or somewhere you can use the abs.

Second is to get a scan tool to do an abs automated bleed. This cycles the valves to release the trapped air.
Almost all full scan tools can do this and it takes only a few mins.
Customer: replied 5 years ago.

Thank you...I believe you're on the right tracks, 'cus there are no leaks anywhere.

 

1. When you say use the ABS you mean apply the brakes at say 10MPH on gravel or ice?

 

2. Where can I borrow a scan tool? Or are they cheap and I can buy one? How can I look it up on the internet? E.g., "ABS Scan Tool".

Yes, exactly, and often the cycling of the valves will cause the air to release.

Any shop can do this or has the tool, and there are no cheap tools for this as the code readers do not work.
Customer: replied 5 years ago.

OK, great thanx.

 

a. In regards XXXXX XXXXX gravel / ice test: If the ABS light is on and the ABS doesn't appear to be working, is this still a valid test?

 

b. Air in the ABS has to go somewhere, so does it get released into the brake system (pipes) and thus require bleeding again?

 

c. In regards XXXXX XXXXX I tried pressure bleeding, but I could never get enough pressure into the resavoir because the fluid would squirt out of the top. So I'm left with gravity or the wife on the pedal. I'd prefer gravity bleeding 'cus the wife's too busy. So my question is, is gravity bleeding 100%? I bleed into bottles with a little fluid at the bottom so not to suck up air.

Great follow up questions.
A, if that does ot release it then it will still need the automated bleed

B. it should go back to the master cylinder but if not then it needs bleeding again.

C. Gravity bleeding is fine once it is release from the system.
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