Need Help - 2000 GMC Yukon XL 1500 SLT 2WD (5.3L) - Brake Pedal Goes to the Floor

[YukonXL813]

I’m hoping someone with GMT800 experience can help because I’m running out of ideas.


This all started after replacing a brake line at the ABS module. Since then, the brake pedal goes straight to the floor with almost no resistance and I have little to no braking.


Here’s everything I’ve done so far:

• Replaced the damaged brake line.
• Installed a new master cylinder.
• Bench-bled the master cylinder before installing it.
• Bled the master cylinder again on the truck.
• Replaced the EBCM (electronic brake control module).
• Swapped the ABS pump motor from a donor truck.
• The original hydraulic ABS block is still on the vehicle.
• Performed an automated ABS bleed with a Snap-on Solus Edge.
• Manually bled all four brakes multiple times.
• Verified brake fluid is reaching the ABS module.
• Cracked brake lines at different locations to verify fluid flow.
• Checked for external leaks and haven’t found any.

Codes I’ve had during this process:

• C0265 (EBCM Relay Circuit)
• C0235 (Left Rear Wheel Speed Sensor Circuit)

I repaired the issues related to those codes. The ABS light is now off, but the red BRAKE light is still on.


Current symptoms:

• Brake pedal goes completely to the floor.
• Little to no braking.
• Bleeder screws only produce dribbles instead of a strong stream of fluid.
• No external leaks.

At this point I’m trying to figure out:

• Could the original ABS hydraulic block have failed internally?
• Is there still trapped air even after an automated ABS bleed?
• Is there another bleeding procedure specific to these GMT800 trucks?
• Has anyone had this exact issue and found the root cause?

Any advice would be greatly appreciated. I’m trying to diagnose it correctly instead of throwing more parts at it.

 

[Fless]

Vacuum assist, or hydroboost? Master cylinders are different part numbers.

When the m/c is bled, how is the fluid output? Strong? Weak? Meh?

I would suspect the new master cylinder; either the wrong one or a defective unit. Suggest testing by finding plugs for the output ports and pressing the pedal to see if it is firm there.

 

[Scottydoggs]

really need a tech 2 or equal scanner to re bleed the ebcm.

 

[YukonXL813]

Vacuum assist, or hydroboost? Master cylinders are different part numbers.

When the m/c is bled, how is the fluid output? Strong? Weak? Meh?

I would suspect the new master cylinder; either the wrong one or a defective unit. Suggest testing by finding plugs for the output ports and pressing the pedal to see if it is firm there.

 

[YukonXL813]

Thanks for the response. It’s a vacuum booster, not hydroboost. The master cylinder is brand new and was bench bled before installation. I also bled it again on the truck and performed an automated ABS bleed with my Snap-on Solus Edge. I honestly haven’t measured the output strength directly from the master cylinder outlets. The next thing I’m going to do is plug both master cylinder outlet ports and see if the pedal gets rock hard like you suggested. If it doesn’t, that would point toward the master cylinder being defective or incorrect. Thanks for the suggestion.

 

[mikez71]

Something I've been hearing lately, that I never ever did before, is to put a stop behind the brake pedal when bleeding.
Something about the piston possibly going too far and catched an edge, tearing up the seal.
Not sure if this is another special chevy thing or what..

I wonder why your brake light is on.. Not the parking brake being engaged?

 

[YukonXL813]

Something I've been hearing lately, that I never ever did before, is to put a stop behind the brake pedal when bleeding.
Something about the piston possibly going too far and catched an edge, tearing up the seal.
Not sure if this is another special chevy thing or what..

I wonder why your brake light is on.. Not the parking brake being engaged?

Thanks for the reply. The parking brake is definitely not engaged. The red BRAKE light has been on throughout this issue, and the ABS light has come and gone depending on where I was in the repair process. I also had a C0235 code earlier for the left rear wheel speed sensor, but the ABS automated bleed completed successfully with my Snap-on Solus Edge.


I installed a brand-new master cylinder and bench bled it before installation. I’ve also performed an ABS automated bleed, manually bled the brakes several times, and just vacuum bled all four corners. I kept the reservoir full the entire time. The rear calipers continued to flow fluid, and the fronts flowed at first but then became intermittent.


One thing that’s really confusing me is that with the engine off, the brake pedal is firm. As soon as I start the engine, the pedal goes straight to the floor every time. I’m trying to determine whether I’m dealing with a defective master cylinder, an issue with the original ABS hydraulic block, or possibly something else in the hydraulic system. I’m trying to avoid throwing more parts at it without knowing for sure.

 

[Fless]

Is the vacuum hose fully connected to the vacuum booster?

 

[YukonXL813]

Yes, it is. I double-checked that the vacuum hose is fully seated on the brake booster and the check valve. With the engine off, the pedal is firm. As soon as I start the engine, the pedal goes to the floor every time. I also removed the master cylinder from the booster (leaving the brake lines connected) and confirmed the booster pushrod is contacting the back of the master cylinder. At this point I’m trying to determine if I’m dealing with a defective master cylinder, an issue with the original ABS hydraulic block, or a problem with the booster itself.

 

[mikez71]

Did you say that the pedal went to the floor after just replacing the broken line, before replacing the other parts?
And was the pedal going to the floor with the broken line?

And do you still have the C0265 code?

I don't trust vacuum bleeding, always seems to draw air in from around the bleeder...
...but my motive bleeder crapped out on me after only 2 uses..

 

[Donal]

Yes, it is. I double-checked that the vacuum hose is fully seated on the brake booster and the check valve. With the engine off, the pedal is firm. As soon as I start the engine, the pedal goes to the floor every time. I also removed the master cylinder from the booster (leaving the brake lines connected) and confirmed the booster pushrod is contacting the back of the master cylinder. At this point I’m trying to determine if I’m dealing with a defective master cylinder, an issue with the original ABS hydraulic block, or a problem with the booster itself.

The bore of the vacuum booster master cylinder is 43mm or 1.34 inches. Confirm that the master cylinder is correct. The bore diameter of the hydro boost cylinder is 1.46 inches.

Be sure that the brake pedal has free play after installation. Be sure that the brake pedal is against the stop and return spring is holding the pedal against the stop.

 

[Fless]

Something I've been hearing lately, that I never ever did before, is to put a stop behind the brake pedal when bleeding.
Something about the piston possibly going too far and catched an edge, tearing up the seal.
Not sure if this is another special chevy thing or what..

Not limited to any particular make/model. Although I think a new MC would be more tolerant of pushing the pedal to the floor, it's a good idea to limit the pedal travel in order to prevent the piston in the MC from going too far. The cylinder bore in older (read: used) MCs will have a spot where the piston has generally traveled, and going past that point can ruin the seal on the piston, causing limited or no brake line pressure.

 

[YukonXL813]

I replaced the original booster in 2024. The bore size aligns with the new master cylinder that I just installed. Would pushing the pedal to the floor still ruin the MC internals?

 

[Fless]

That's hard to say. Testing it with the outputs plugged would likely tell.

Hopefully when you are testing you're applying vacuum to the booster. It needs the engine running to do its job.

 

[Donal]

I replaced the original booster in 2024. The bore size aligns with the new master cylinder that I just installed. Would pushing the pedal to the floor still ruin the MC internals

I just reread every post from the OP. Two conclusions, 1-air in system, 2-brake pedal does not have free travel to unload the rod between the between the pedal and master cylinder piston.