Losing Brake Pedal Pressure

Howdy,
My '05 Yukon XL has an issue where the brake pedal loses pressure when at a stop. To the point where it starts rolling through the brakes and the "Service Brakes" alarm sounds. I recently changed all the steel brake lines, master cylinder and bled the lines a couple times.

That thread is here: https://www.tahoeyukonforum.com/threads/mushy-brake-pedal-after-new-brake-lines.110137/

I haven't had a chance to look in to it too deeply yet and I have not seen any puddles of brake fluid on the ground either. So, I'm a little stumped on where to start. Any ideas?

Thanks in advance,
Lord Snax

sounds like you need to bleed the master, then re bleed the calipers.

i like to pump the pedal up, then crack the line nuts off the master till you no longer hear air spitting out, you can wrap the nut and wrench with a rag to keep from making a huge mess.

Air in the lines or master. I love the Motiv pressure bleeder, makes it easy.

Probably a bad master cylinder. What brand did you use? New or Reman?

Soft pedal or creeping pedal?

If you pump and hold, do you still lose pressure?

Could you clarify if your pedal is creeping down, or creeping up as in losing power boost?

If pump+hold holds pressure indefinitely

If it still doesn't hold pressure:
- Inspect system for external leaks. For better or worse this means tracing every brake line from master cylinder, through abs, to the caliper and wheel it goes to. Fluid on ground would be a positive indicator for a leak, but the lack thereof is not a cause for rejecting a leak.
- Inspect brake fluid. Has the level changed since you performed the brake service?
- Remove the vaccuum line from the master, inspect internally to see if there's any brake fluid inside it, pump the pedal, then see if it still creeps. Pedal will be firmer when doing this, be mindful not to confuse that with an indication that it's problem solved. Inspect the inside to see if there's a leak into the booster

AIR in the ABS block ?



oh...I just read your thread - you still used a OBD Tool do beed the ABS block.. so i would go through the MC. Even if it is a AC Delco . I just ordered one from RA...AC Delco OEM
MADE IN CHINA hope that "he " will work.

Alright, I had a good chance to look at my Yukon today after work.

After spending a couple of hours trying to find the leak and it turns out it was internal.

A shout out to @swathdiver for suggesting the master cylinder. It turns out the one I bought from Autozone was leaking internally and causing me all this headache. Luckily, Autozone honored the warranty and replaced it for free. Now my Yukon stops on a dime and gives you $.09 change.


Thank you all for your help. This saved me from another car payment!

the one you bought from Autozone.... was it a AC Delco ? Or A1 Cardone... just for the records

brasil said:

the one you bought from Autozone.... was it a AC Delco ? Or A1 Cardone... just for the records

Click to expand...

It is a new Duralast master cylinder. It looks like the one I originally bought had a defect. The replacement works perfectly.

Lord Snax said:

It is a new Duralast master cylinder. It looks like the one I originally bought had a defect. The replacement works perfectly.

Click to expand...

That's genuinely pathetic.

I don't like it, but I at least appreciate the concept of acceptable failure rate, particularly when things are shopped down to the lowest price. But a master cylinder? Or anything in the brakes? That's just wrong....