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.