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2008 Yxxx XL Pedal goes to floor solution

Brake Bleeding
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2008 Yxxx XL Pedal goes to floor solution

Postby Westlotorn [OP] » Jan 22 2021, 10:00am

I wanted to share, I bought a 2008 Yxxx XL, I bought it not running, AFM lifter collapse issue. Got the engine repaired and found I had no brake pedal, it goes to the floor, you can pump it up with several pumps but let it relax for a while, 1 minute and the pedal goes to the floor again. OK, I figured do a complete brake job and the problem will surface. All new brakes, complete flush of the system brake fluid, all new Dot 4 installed. When finished I check the brake pedal, exactly the same???
Next, I hook up my Launch 431V+ scan tool and do the ABS bleed to the system, now my pedal is worse, I use the Launch to check all the ABS valves, return and supply for each wheel. I find if I close the supply valve for my Pass side front brake I instantly get a good brake pedal. Wow, did not expect that. So I flush the pass side wheel bleeding it a lot more, maybe a quart in just this wheel. I activate and close those valves many times. Hoping something is stuck in there and hoping to fix it. No luck but pedal is a little better.
I will share I have done many brake bleed jobs on many cars, usually I start at the pass rear, then driver rear, then pass front and finish at the driver front. Usually Pass rear I flush a lot of fluid till it is clean because this is the longest run, then the driver side rear takes much less time to get clear fluid etc with the front wheels. This program did not work to bleed this GMC. I still had no brake pedal unless I pumped it up. The front wheel lines are still long runs on this vehicle so in the future I will spend more time bleeding these lines like the rear. Much different than a non ABS vehicle.

Being experienced I decide the master cylinder is not building pressure unless it is pumped. (ok, not so smart) I replaced the Master Cylinder. Bled everything again. Still no hard pedal but it is a little better???

Now I crawl under this vehicle and trace the ABS lines to see how they run. Basically the Master pushes fluid down to the ABS Controller which is kind of under the driver seat by the frame. Then all the fluid exits in its own dedicated line to each wheel from the ABS controller. OK, the light is coming on. Normal bleeding is complicated by the longer lines in the ABS system.
I go back to the drawing board. Bled the pass side front brake a lot more because closing this run with my scan tool gave me a good pedal. Ran about a pint at least at just this wheel. I did this because knowing the location of the ABS controller and the distance from it to the front pass brake. Just doing this and then using the launch again to cycle the ABS bleed program and my brake pedal was nearly good. 80% better than at the start. Knowing this I know the system still had some air trapped. Now I went to the Pass side rear, started bleeding that wheel again and I got a little more air out. Pedal was better yet. Repeated this on the last wheel, driver front and the pedal is restored to factory perfect. Pedal moves about 1" and is now hard. I am finally happy but it was a long battle. I have read that reverse filling the system eliminates this type issue with ABS systems but I don't have that tool in my garage to reverse fill. I used a full gallon of brake fluid in this truck flushing. The very odd thing to me is on the last bleeding of the rear brakes I got some really dirty fluid out after they had been bled till clean prior. My guess is this was trapped in the ABS system along with air that I did not get out in my first brake bleed attempts. I hope this helps some other poor soul fix your truck. I have never worked this hard on a brake bleed job. This is not uncharted ground, I have bled 9 vehicles this year with compete fluid flush on all of them. This one was my pain in the neck. I never should have replaced the master cylinder, the old one was probably fine. Since I did not own this truck prior I have no idea what kind of brakes they had before the engine failed.
If it was not clear in my late night writing, I did use the ABS brake bleed with my scan tool at least 4 times in this process thinking that would fix my issue, I am sure it helped but moving a lot of fluid through the system seemed to fix it, once I knew the major problem was at the pass side front it allowed me to focus on that line and everything finally got fixed.
I pumped the brake pedal slowly and held it at the down and up positions each time trying to avoid mixing any air bubbles.
If someone tells me a 10 minute solution to this job I might just cry, I'm happy with the result or finished product but man what a pain I need an easier way for my next job like this.
I did read about guys braking hard on ice or gravel to make the ABS system repair itself, I did not have that option to try, no gravel or ice, not even a wet rainy road to try that method.
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Westlotorn
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Re: 2008 Yxxx XL Pedal goes to floor solution

Postby mikew » Jan 22 2021, 10:25am

I have hooked up my scan tool with ABS bleed. It tells me to start at driver's front wheel, I assume this is the one farthest from the pump.

This is as far as I got, I haven't performed the bleed yet.
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Re: 2008 Yxxx XL Pedal goes to floor solution

Postby Westlotorn [OP] » Jan 22 2021, 8:05pm

My tool is the Launch X431V+, a nice tool that offers quite a few options to help fix these cars but costs about $1300, not a cheap device but it help. Like yours it instructs which wheel to bleed and has you pump the brake pedal with that specific wheel bleed screw open when it finishes you close that bleed screw and move to the next as indicated. It starts at the driver front, then pass front, pass rear and finishes at driver rear. The ABS pump runs and pumps fluid at each wheel as this test is performed. It helped for sure, as stated I did this procedure at least 4 times along with the traditional bleeding to get the air out of this System. It did work eventually and I really wish I had not bothered to change out the master cylinder.
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Re: 2008 Yxxx XL Pedal goes to floor solution

Postby mikew » Jan 23 2021, 12:07am

mikew wrote:I have hooked up my scan tool with ABS bleed. It tells me to start at driver's front wheel, I assume this is the one farthest from the pump.

This is as far as I got, I haven't performed the bleed yet.


Do you feel the ABS bleed replaces all of the fluid in the system?
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Re: 2008 Yxxx XL Pedal goes to floor solution

Postby Westlotorn [OP] » Jan 23 2021, 10:02am

Maybe, I am trying to remember exactly how much I bled with just the ABS bleed using the scan tool, my bleed container holds a pint and I filled it doing all 4 wheels. That is pretty close to the full system bleed?

There must be a preferred way to bleed these systems that I have yet to learn, I eventually got it bled well but spent way too much time on the attempt.
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Re: 2008 Yxxx XL Pedal goes to floor solution

Postby Chevy-SSChevy-SS is online! » Jan 25 2021, 10:36pm

Great writeup, thanks! I have owned numerous Chevy trucks, the last two being a wonderful '98 K1500, and now my current gem, a 118k mile, runs-like-new '03 Silverado which I just love.

The ONLY issues I have had with both of these trucks is getting good pedal after brake work. It has been an INCREDIBLE amount of work and aggravation, with bleeding and ABS cycling (I use AutoEnginuity on a laptop), then more bleeding, then more ABS cycling, rinse and repeat, rinse and repeat....arghhhhhhh :twisted:

I have been doing brake jobs for many years. At one point (in 70's & 80's) I owned two service stations, so I've probably done thousands of brake jobs (mostly on non-ABS cars though). But I've never had bleed issues like with these trucks, LOL.
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Re: 2008 Yxxx XL Pedal goes to floor solution

Postby Westlotorn [OP] » Jan 27 2021, 2:31am

I keep hoping someone will show us the quick way to flush these systems. The 2008 I just did had very dirty brake fluid in it so I had to do it but never expected this much work. Maybe one of the Motive pressure bleeders would have done this job complete and fast?
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Re: 2008 Yxxx XL Pedal goes to floor solution

Postby Chevy-SSChevy-SS is online! » Jan 27 2021, 6:53pm

Westlotorn wrote:I keep hoping someone will show us the quick way to flush these systems. The 2008 I just did had very dirty brake fluid in it so I had to do it but never expected this much work. Maybe one of the Motive pressure bleeders would have done this job complete and fast?


I've been using a Motive bleeder for years. It is terrific..... but did not seem to help with the Silverado issues. :?
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Re: 2008 Yxxx XL Pedal goes to floor solution

Postby Westlotorn [OP] » Jan 27 2021, 7:24pm

The strangest part of getting my truck properly bled was after flushing several times, probably 3 quarts flushed into this job, and yes, I did it all the way around, all 4 brakes, after all that when I finally got the ABS pump flushed and spit out the last of the air I got some really dirty fluid out of the system. I have never seen a system trap old fluid, I would think it would have flushed right away like all the other cars I have flushed. Somehow it traps fluid inside the system and in my case it also trapped some air which gave me the very soft brake pedal.
I have been using a bleed container that has a one way valve so fluid and air can go in but not out. You can also hook up a vacuum pump if desired, I really like this tool it speeds up brake bleeding a ton normally.
I did not use vacuum on the Silverado, just the brake pumping method. Press down, hold for 3 seconds or so, let up slowly, hold for 3 seconds to allow refill and press down again, always steady speed not fast to avoid mixing up air in the system.
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Re: 2008 Yxxx XL Pedal goes to floor solution

Postby Chevy-SSChevy-SS is online! » Jan 27 2021, 9:23pm

I still feel like there's some air (or something) trapped in mine. Sometimes my brakes are perfect, absolutely 100%. But most of the time, my brakes are mediocre, like there's a little air in system. It (the brake system) fluctuates with a mind of its own. It's truly frustrating. I'm just glad it's never happened on customers cars, LOL!!! But I'm very careful about what jobs I do nowadays, I try to stay away from stuff that involves ABS.
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