Brake Problem on a 97 GMC K1500 Sierra SLT

tommy99 · July 21, 2014

I have a 97 K1500 and the brake pedal would go to the floor when I pressed it. I did a full brake job on the front (calipers, pads, rotors). The rear shoes and drums were in good shape so I put them back on without changing. The brake fluid was black so I bled the reservoir and all four brake lines in correct sequence using a Mityvac 6835 brake bleeder. I bled all the lines two times just to make sure I got all the old black fluid out.

Once I got it back together the pedal would only hold pressure briefly and then go to the floor again, and now the red check brake light will go on and off once the pedal drops.

Any ideas?

Thank you

Squirrely · July 21, 2014

Did you adjust the shoes on the rear out as far as possible? If not, the pedal can go to the floor.

tommy99 · July 21, 2014

Yes, I adjusted them all the way up to where they had a decent amount of drag when I put the drums back on. They can't be adjusted from the back while the drum is on so I got them up as high as possible before I reinstalled the drums. I briefly get pressure in the pedal but then it drops and the check brake light comes on.

Dutchtouch · July 21, 2014

Sounds like your master cylinder may have failed or you have an incredible amount of air in you system.

tommy99 · July 21, 2014

I had the soft pedal problem before the bleed too. I don't think there is a lot of air in the system since I was pulling it through the lines using vaccum. I let each line bleed for about one minute and I sealed each bleeder screw while the machine was still pulling fluid through the line. I guess the master cylinder is the only thing left.

Edited July 21, 2014 by tommy99

Squirrely · July 21, 2014

Then I would say the master cylinder is gone.

tommy99 · July 23, 2014

So I bench bled and swapped out the master cylinder. Vacuum bled all of the lines again with about 45 secs of vacuum on each line. I'm still getting the red check brake light and the pedal is soft and going to the floor. I'm not getting any leaks in the lines or bleeders. Has anyone had a similar experience? Is it possible there is some air trapped in the ABS? Would the yellow ABS light come on if there was air in it, because I'm not getting the ABS light.

Any ideas?

tommy99 · July 24, 2014

It was two bad wheel cylinders. I was re-bleeding the brakes and one of them finally popped. Both wheel cylinders were slightly leaking but the dust boot was catching the brake fluid before it could pour out.

Duelin · July 24, 2014

Thanks for the followup.

Squirrely · July 26, 2014

word

lodihoseman · August 10, 2014

Brake fluid absorbs water and should be replaced every couple of years. Water is heavier than brake fluid, so the water collects in the wheel cylinder. The water causes the cylinder to pit, and eventually leak.

tommy99 · August 10, 2014

Brake fluid absorbs water and should be replaced every couple of years. Water is heavier than brake fluid, so the water collects in the wheel cylinder. The water causes the cylinder to pit, and eventually leak.

I bleed my brakes every year, but this is a friend's truck, and it was heavily neglected. I doubt that fluid had ever been changed. I disassembled both wheel cylinders and they were pitted and scarred.

To add to this topic.. The ABS system was also damaged. I'm guessing the damage was caused by the dirty brake fluid since it seemed to ruin everything else it came in contact with. The replacement ABS unit will be here in a couple of days. Once I get it swapped out, and everything is working, I will disassemble the old ABS valve body to see how bad it looks. I can post pics if anyone is interested.

Edited August 10, 2014 by tommy99

tommy99 · August 23, 2014

Wow, so this brake job saga is finally over. I replaced the master cylinder, ABS unit, rotors, pads, calipers, drums, shoes, springs, and wheel cylinders. I would get a solid pedal when it was up on jacks but as soon as I started driving the pedal would drop down and start pulsating. I wasn't getting any check lights, and I bled the entire system and bench bled the master cylinder multiple times, but every time I would get on the road the pedal would pulsate up and down as soon as I pressed it.

Well I finally figured out what was wrong after weeks of working on this thing. The dirty brake fluid didn't just destroy the rubber in the wheel cylinders, it also started eating up the rubber flex lines from the inside. They got so weak that as soon as I would apply the brake and build pressure in the lines they would start expanding and collapsing. The fluctuation of pressure in the lines would make the pedal pulsate up and down. After changing all of the flex lines I got a solid braking system.

Daryl Z71 · August 23, 2014

Good to hear you got it fixed up.

kilbornf · August 25, 2014

Good to see all the knowledge here on brakes.i have the same truck but mine is a diesel.Brakes are okay but should be much better.They will not skid the front wheels on hard braking,only back will skid if hard braking.New front pads,calipers and rotors.Rear has new drums,shoes and wheel cylinders. To me you have to push peddle with to much effort to make truck stop.All that is left is Hydrabooster on a diesel and the Master cylinder.No visible leaks on either unit.What you all think?I think there is not enough braking pressure to front wheels.