Soft brake pedal: Suburban at 307k miles

[tleed]

I have a '96 K1500 diesel Suburban. The brake pedal has been slowly going soft. I had it into a shop recently that replaced the driver's side rear brake line, but that didn't fix the soft pedal. Then when I got it home, I noticed the brake lines around the left front were extremely rusty, too, so I also replaced them & then bled the system again. I ran a lot of fluid through to flush everything out. I also had to change the driver's front caliper because the bleeder valve broke off, and the right rear bleeder valve stripped, so it got a new wheel cylinder. I have the JD7 code 13" rear drum heavy duty hydraulic brake system.

 

But the brake pedal is still soft. I'm wondering what to do next.

 

Is it time to go after the master cylinder and/or vacuum booster? Do I do one first, or both together? And how do I tell which one to change first?

 

I'm showing 307k miles. I've only had it since 224k miles and I don't know if the latter 2 items were changed prior to my getting the Suburban. But I know I didn't change them between 224k and 307k.

 

Incidentally, it appears that the brake lines go into another box on the fender. I think it said Kelsey-Hayes on it. I assume it was part of the ABS system. I made no special effort to bleed it, although I'm sure air got introduced there when I replaced the lines. I just assumed you bled at the wheels to remove all the air: driver's front first, then passenger's front, then driver's rear, then passenger's rear.

 

Suggestions? What's next?

 

Thomas

[O_J_Simpson]

Bleed in entire system again. You have a bubble trapped.

[00Silverado4x4]

That other box may be the portioning valve which definitely will need to be bled. Not sure the best way to do that. Also make sure that your rear brakes are adjusted properly. The rear brakes may need to be adjusted up a bit more.

[2018GMC]

To bleed abs on my 97 k1500 truck, I raised all 4 wheels off ground. Put in 4hi. Accelerate to 50mph. Stomp on brake pedal. Abs will go crazy pulsating. I did this 4 or 5 times while on jacks. After that my brake pedal is firmer than it's ever been. I also noticed my once clean fluid in master cylinder was dirty again because I had bleed the abs lines and abs module. There is a special tool or computer to bleed abs system. But this method worked great for me. BTW Installing drilled and slotted front rotors made a huge difference in braking power. Only downside is increased brake dust on front wheels.

[tleed]

My rear brake drums were super snug going on, so I don't think they need to be adjusted.

 

I pushed a whole quart of brake fluid through the lines bleeding them. I had a friend step on the brake to help. I put a clear hose over the bleed screw, but I'm not sure it did much more than show the bubbles coming out.

 

We bled like this: I'd open the screw, he'd push the pedal to the floor & hold it, I'd close the screw, he'd release the pedal. He watched the fluid level & made sure it never dropped too far. At each wheel, large bubbles would come out at first, and then they would diminish to a tiny stream. None of them got to zero bubbles, and I had a feeling they never would.

 

I've been told that that tiny bubble stream can be air coming from around the bleed screw. If that's wrong, then how long should it take to get past that tiny little bubble stream? Is there something wrong with my method?

 

Also, I don't mind trying the 50 mph-on-jack-stands method of bleeding the ABS lines & module. But where does that air go when it's removed from them? Downstream to the brake lines that have to be bled again? I'm not surprised that the ABS module & lines might have air in them, since I had to pull the brake lines loose from the ABS unit to replace them. But is that any different from activating the ABS by driving at 50 mph and stomping on the brakes?

[2018GMC]

I had air in my system after replacing front calipers. The air seemed to go to the master cylinder when cycling abs. I would cycle abs a few times. Then Bleed starting at passenger read then moving up to driver for final bleed. After bleed cycle abs again and you should be good to go!

 

 

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[2018GMC]

The abs only gets cycled when it detects wheel lock. This rarely happens when all 4 wheels on dry pavement. When truck is on jack stands wheel lock happens a hell of a lot which cycles the crap out of he abs pump = groaning and pulsating brake pedal.

 

 

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[00Silverado4x4]

Bubbles streaming is definitely a problem. You need to get all the air out. If you are only opening the bleeder screw just enough to release fluid then I doubt you are getting air from around the screw. Unfortunately you need to double check everything in your brake system for any possible leaks and then keep bleeding until you have nothing but solid fluid coming out of the bleeder screws.

[O_J_Simpson]

None of them got to zero bubbles

Ans there is your problem. You have to get all the air out of the system.