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.