Brakes refuse to bleed properly

715Hunter

715Hunter

Member
Jul 28, 2022
83
16
8
Missouri
We've got an 04 LB7 work truck that has been giving us major brake issues. It drives fine, but when really slamming on the brakes they just won't lock up. No ABS engaging, just feels like 75-80% application with the pedal on the floor. The pedal is very spongy too. I've done the power brake bleed with a scan tool more times than I can count, replaced the master cylinder twice, replaced both front rubber lines, replaced the hydrobooster, and the power steering pump is relatively new. The problem seems to be in the front half; unhooking and plugging the front line makes the pedal firm, but vise-versa it's still soft. We've bled all lines both directions many times, with quarts of fluid to guarantee no trapped air bubbles. I've also taken it to two good local mechanics,and neither could find anything wrong.
The only component left to replace is the ABS module, but that's a lot of work for not knowing what's wrong for sure.
Any advice or knowledge hugely appreciated!

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715Hunter

715Hunter

Member
Jul 28, 2022
83
16
8
Missouri
2004LB7 said:
Maybe describe your bleeding process so we can see if you're doing anything wrong.
Click to expand...
We've done it the old fashioned way: Crack the bleeder, someone else pushes the pedal, close it went the stream gets small, repeat. Then power bleed for the ABS, manual bleed again. This improves it for a few days, then the pedal gets soft again.
Vacuum bleed with this:
Both methods would keep producing the occasional bubble, no matter how much we sent through it.
Lastly, we tried bleeding it backwards, pushing fluid up from the calipers to the reservoir. This worked the best so far, getting all the air out and making the brakes feel semi-decent.
 
715Hunter

715Hunter

Member
Jul 28, 2022
83
16
8
Missouri
Well, we may have accidentally fixed it. After the last round of new parts, including a new master cylinder and a different ABS module, it actually brakes pretty well. Nothing done different with the bleeding, but it bled out much easier. It will now lock up the front tires on asphalt if you smash the brake pedal. I haven't towed with it yet, so don't know if it's 100% "right".
The worst thing about this issue is that most people would drive it and not notice anything wrong. Only when heavy braking was really needed did the problem show up.

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