1999 Yukon Denali 4x4 can't fix soft brake pedal

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lisach

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I've read some similar threads here but still not sure what to do. 1999 Yukon Denali 4x4 Front Disc Rear Drums 147K miles. I've had a soft pedal for a long time. Ever since I got the car 2 years ago. Last month after replacing the front brake pads, I got really serious about finding the solution. So I've replaced the calipers (thought it wouldn't hurt. I pull trailers on mountains a lot). and the flex brake lines for the calipers. flushed and replaced the whole system brake fluid. Bled the brakes. 3 times. At last it began to grab pretty well, but still soft.

Decided to replace the master cylinder. Bled the MC on the car. Not a bench bleed. Bled the brakes. Better.... but still not the way it should be. Sitting in park the pedal goes all the way to the floor. Weird thing is the brake pedal makes a sound. Don't want to call it hissing cause its not, Don't want to call it squishy cause that's more of a feel, but it really is a squishy "sound". With engine off the pedal pumps to hard, turn it on and the pedal goes to the floor. Bled the brakes again and still no different.

Saw a suggestion to use a 2x4 to prevent the pedal from bottoming out. Not sure about why this is a problem, but we definitely pushed the pedal to the floor.

I really believe there is still sir somewhere in the system and not sure what to do next. New pads, new calipers, new flex lines, new brake fluid, new master cylinder...... bled the Master Cylinder and the brakes multiple times.....

btw... power stop drilled & slotted rotors with power stop pads, AC delco calipers, Duralast master Cylinder

Suggestions please?
 
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east302

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Have the drums been adjusted? That usually helps the most.

You might revisit that MC bleed, pointing it down. Or, I’ve heard of people jacking the back up, loosening the mounting bolts from the booster, pointing the master cylinder down and lightly using the brake pedal to push the MC piston. I’ve never tried that, though.

I’ve had the best luck with a pressure bleed using a Motive bleeder (looks like a garden hose sprayer) with a BA14 adapter:


8731573C-BD7C-46C7-93AD-E4BE62778F8F.png

D5E27D47-A293-4F11-9C35-ED6C7DC362E4.jpeg

You’d connect that to the Motive line with an automotive x industrial quick disconnect. Or a “universal” one works just as well. Add a ball valve in the line to help control spills.

AAAD59CB-A830-4806-BC09-1D179F0001D4.jpeg

And last, you can run an ABS brake bleed with a good bidirectional scanner like a Tech2 or probably a SnapOn. That is an automated sequence that purges air from the ABS unit into the brake lines. You’d follow with a normal brake bleed. If you don’t have a scanner, a shop would probably do it if you told them what you’re looking for.
 
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drakon543

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if you broke the lines and didn't rebleed the rest that also might be your problem. something really stupid ive had where i thought my lines were tight enough because i didn't have any leaks but i was wrong. they weren't leaking but every time you released the pedal it was pulling air into the system. unless the slave cylinder went bad you will probably be fine after a full bleed.
 

exp500

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It definately works different than many older(70's and earlier) in that usually takes about 2 quarts of brake fluid to bleed completely. Good luck!
 

B-train

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@drakon543 good point. You have to really crush the washer on the caliper mount for the hose. It should have 2 copper crush washers. It feels like you want to break it off - takes a calibrated arm, but pull with good steady force. Hard to explain over text, but hopefully you get the idea. You want the ridges on the line to squished into the copper.

Also, i second the ABS bleed procedure. Those things can get airbound. If you don't have a scanner, try and find a road with loose gravel and do some ABS stops, that may work out the trapped air.

Also, with rear drums, if they have a lot of travel, they will make a low pedal. It might be worth a look with 147K on it. If they look good, rinse down with brake cleaner. Then make sure the self adjuster spins freely. Ratchet the adjuster until you feel/hear a slight drag when rotating the drum. This will decrease the amount of travel before the leading shoe engages.
 

Eman85

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Fluid cannot be compressed, first thing to know about hydraulics. To troubleshoot where your problem is if you think it's hydraulic is to isolate. You can buy plugs for the master cylinder and screw them in, pedal should be like a rock with them in if not you found your problem. You can then attach 1 section at a time and see where the sponginess is. Of course this is after you know for sure all of the mechanical sections are working properly and rears are adjusted. You can isolate front and back by plugging or clamping hoses.
All of these have crap brake pedals even when working good. A hack is to install a later model MC, you can google about it. I did it on my K2500 and pedal is much better.
 

j91z28d1

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most likely it's the abs module. they get old and bypass internally.

it's worth the 200$ to pick up a tech 2 for these trucks. it can access these old abs, pull codes from them and run the motors. even adjust the tire size for the abs. it doesn't change the speedo or ecm, but how the abs works.

I forgot what the manufacturer name of these abs is, it's been a few years since I looked into this low pedal on my 96. it's common across everything gm put it is. but look on it, it should have the name, google it and read up on it. I believe they can be repaired. but most of the time the best mod is to just bypass it. there's threads around for that too. and then bam, normal hard pedal.

years ago my mom has a 96 blazer and it had the same abs module manufacturer and same crap pedal.
 
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lisach

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most likely it's the abs module. they get old and bypass internally.

it's worth the 200$ to pick up a tech 2 for these trucks. it can access these old abs, pull codes from them and run the motors. even adjust the tire size for the abs. it doesn't change the speedo or ecm, but how the abs works.

I forgot what the manufacturer name of these abs is, it's been a few years since I looked into this low pedal on my 96. it's common across everything gm put it is. but look on it, it should have the name, google it and read up on it. I believe they can be repaired. but most of the time the best mod is to just bypass it. there's threads around for that too. and then bam, normal hard pedal.

years ago my mom has a 96 blazer and it had the same abs module manufacturer and same crap pedal.
I'd like to thank everyone who gave their input to this thread. I fully intended to follow-up with this. I hate threads where posters don't follow up with results. But the very day I was scheduled to have an ABS bleed done at local shop, my 96 yr old mother went into the hospital, then to hospice, then home. She is still with us against all odds and I have to put a pin this until I can finish. Haven't driven the car 20 miles since.

I will post results when it's complete.
 

OR VietVet

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I agree with @B-train about basics first. Check and adjust the rear shoes. If they travel too much before drum contact, that is likely the problem. If they travel too much, the park brake likely does not set right. Also, on that age rig, check the three backing plate contact points, for each shoe, where the edge of the shoe rides and make sure that backing plate contact point is flat with no ridges. If has ridges, that can cause the shoe movement to be suspect. Is there any fluid leakage at the wheel cylinders? Leakage at brake lines that feed the wheel cylinders? The 2x4 is used to keep from bottoming out the brake pedal when bleeding the master cylinder/brakes. If bottoms out, you can damage/flip a seal internally in the master cylinder. Also, that Tech II is great for bleeding ABS systems.
 
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lisach

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I'd like to thank everyone who gave their input to this thread. I fully intended to follow-up with this. I hate threads where posters don't follow up with results. But the very day I was scheduled to have an ABS bleed done at local shop, my 96 yr old mother went into the hospital, then to hospice, then home. She is still with us against all odds and I have to put a pin this until I can finish. Haven't driven the car 20 miles since.

I will post results when it's complete.
Well. Here I am back after several miserable months of absence from life in general, but with results of brake issues.
Yes. My car is 24 years old. But it's the 3rd version of this model I've had since 2005. I love these cars and I bought this one in 2020 with 111K miles as 2nd owner. So I'll be keeping it for a while.

Anyway, after replacing the brake lines, the calipers, the master cylinder, flushing the fluid and bleeding everything multiple times, I still had a soft brake pedal. I worked on the rear drums , adjusting the shoes and doing everything that could be done there . Still .. the pedal was no better. Finally I did take it to a shop to have an ABS bleed.

That was the ticket. The brakes are now about as good as they're going to get understanding that I have drums on the rear.

Thanks to everyone who gave suggestions. This is the best place to get solid advice.
 

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