1999 Tahoe, soft brake pedal

[99brokeTahoe]

I'm going to try and swap the whole thing. I bought a 97 Tahoe a few months ago with a bad trans.

 

[99brokeTahoe]

Sorry for the long post, but because it's brakes, and there are many variables, I think the details are important. Since this is brakes, let me start this off with "this is just information, I'm not telling anyone to try this, since this brakes, and if it's not right, you could cause property damage, or hurt/kill someone. Seek a professional for help if you have any doubts about your brakes". I swapped the whole ABS module, then hooked my scan tool to the truck, and it wouldn't communicate. My truck is 1999 K1500 Tahoe, donor was 1997 C1500 Tahoe. All the electrical plugs were the same, from outside appearance, they looked the same, but it wouldn't communicate with my scan tool. So I swapped the black box, electrical part onto the donor manifold. I was going to start the bleeding procedure, but looked in my master cylinder, and seen a lot of trash that wasn't there a week or so ago when I replaced the master cylinder. I think all the trash came out of the ABS manifold. So removed the master cylinder, and dumped it out for fresh fluid. After learning that the donor ABS module wouldn't communicate, I decided to take it apart. The electrical part is only held on with 4 torx screws, and the manifold is sealed from it. The module must just be a circuit board and the coils for the sealed manifold. I swapped my old module on to my donor brake manifold. Scan tool communicated, so I started by manually bleeding the master cylinder first, then each wheel, starting farthest away, to closest. Then I did the automated bleed procedure, and bled all 4 wheels the same as before. I did this 3 times, because I knew I had opened the system up, more than ever before, so I knew it was going to be difficult to get the air out. Brake pedal felt better after that (before I started this, my brake pedal with the engine running, would slowly go to the floor, no way you could panic stop it, and the front wheels would lock up before you got to the floor) so this was an improvement. I test drove it, and it feels much better, not 100%, but much better than it was. Better than just unplugging the ABS module. If you try this, be prepared to bleed the brakes till air stops coming out. I used a scan tool, and locked the wheels up, then manually bled them. I've done this 4 times, and the forth time, it stopped improving. The truck stops much better than it was, and my pedal isn't going to the floor like before. Just to "recap", I replaced my front hub bearings on both sides, and contaminated the wheel speed sensors with antisieze. I didn't have a problem immediately after the hub bearings, I think the heat from driving made the thick antisieze more liquid form, and it slowly coated the tone rings. Then, it started coating the sensors. I only found this, because a member told me to use the scan tool to bled the brakes. Never had done that, never needed to. Thank you to that member, because it wouldn't bled because it had ABS speed sensor codes, and there's a TSB on them. That made me tear it all the way back down to the sensors, that's how I found that. I think that the 290,*** miles, and the speed sensors making the ABS module try to activate damaged something in the ABS manifold. The original problem was at low speeds (below 10MPH) the brake pedal would feel like it was binding up, you knew you had more pedal travel and the truck was still rolling, then the brake pedal would go down some more, and you would stop. It didn't feel like ABS. I misdiagnosed it for a master cylinder, or booster, because it didn't feel like ABS. After replacing both of those, no change, except when I put the booster on, the brake pedal started going to the floor. Tried bleeding it all different ways, nothing helped. Finally, tried unplugging the electrical plugs from the module, and that helped, a lot. I had 97 2WD parts truck, so I swapped the ABS manifold over. This made it much better. Very difficult to bled though. Now I'm going to try and find a new/reman ABS module/manifold, because I would think the valving would be specific to the vehicle.

 

[Eman85]

I have a question, when you replaced hubs or front brakes did you push the caliper pistons back without opening the bleeder screws?

 

[99brokeTahoe]

I have a question, when you replaced hubs or front brakes did you push the caliper pistons back without opening the bleeder screws?

Yes, I did. I've heard that you need to do that, instead of just forcing the fluid back though the ABS module, and MC. I guess I have just been lucky, and never had a problem before. I think all that trash came from doing the automated bled procedure. Other than the hard lines and ABS, everything else is new within the last 2 years, and when I would replace a component, I would manually bled the system till I got clean fluid. I think I will start opening the bleeders to push the calipers back now, after this.

 

[Eman85]

People don't realize the dirt from the cast iron calipers and other metallic parts in the system that they are forcing back into the valves and the MC. Brake fluid attracts water and causes corrosion which is why there is a brake fluid flush as a maintenance item on many vehicles. Most get away with it, you unfortunately didn't

 

[99brokeTahoe]

People don't realize the dirt from the cast iron calipers and other metallic parts in the system that they are forcing back into the valves and the MC. Brake fluid attracts water and causes corrosion which is why there is a brake fluid flush as a maintenance item on many vehicles. Most get away with it, you unfortunately didn't

I guess that's possible, but the calipers and lines are maybe a year old. And when they were installed, I made sure to bled the brakes till I had clean fluid coming out. The wheel cylinders and shoes are a little older, but I thought I flushed the system pretty well when I installed the new parts. I really think I caused the problem with the antisieze on the speed sensors. There's a TSB on that. All that trash came from doing the automated bled procedure.