Hello all,

I call my technicien friends in the US because I can't fix an issue with the brake system on my Hummer H2.
Six month ago, the brake pedal became spongy and finally 10 days later went to the floor !

Don't worry Fabian, the truck has 120.000 miles and almost no doubt that the master cylinder seals become "tired" !!!
The master cylinder has been replaced by a brand new AC Deco and ...... doesn't fix the problem at all !

Problem description is :
=> I bleed the brake system and the pedal is fully hard.

=> I drive 15 to 20 days and progressively the pedal become spongy and finally goes to the floor !

=> When I bleed the system, I get air in the 4 lines !!! and it needs between 2 to 3 pedale strokes on each wheel to get air (air does looks to be close to the calipers).

=> I have never seen any leaks (nothing noticeable) and the oil thank level looks to be stable !

- Do I get a bad master cylinder or something wrong when installing it ?
- Does it the ABS bloc that failed ?

Please do you have any Idea to help me.

In belgium nobody has any experience on this vehicule, so I have to get it by myself but this time..... I'm really stuck !

Thank you for your help

Fabian

PS : Sorry for my ''bad'' english. I do my best but it's a really technical vocabulary and no sure to find right words .

 

You still probably have air in the ABS module. You will need to find someone with a scanner that will allow you to activate the solenoids in the ABS pump to get rid of the air in the system.

p.s. Your English is great.

 

Thank you Phil for your answer.

I don't know if air still remain in the system because after bleeding, the pedal is fully hard and the braking system run very well but...... day after day, air comes in step by step and the pedal become spongy !

I already bleed a bit less than 15 time !!! and always same behavior (not better, not worse).

I become crazy !

 

A couple things come to mind, our trucks have full floating calipers which should prevent runout from pumping air into the system but If the rotor runout becomes bad enough they can pump air into the system. The seals are designed to keep oil in and and everything else out but they will allow air in if the runout is bad enough.

This was a major issue back with the C3 corvettes which did not have floating calipers so any amount of run out would cause this exact issue with them.

This usually isn't an issue with floating calipers,

make sure that all the calipers are floating, I've seen some that where stuck in place. I would check the runout of the rotors by removing the calipers from the mounting bracket and using a dial indicator placed against the face of the rotor then spin the rotor and see how much runout it has, it should be less than around .005" in most cases. I did not look it up for our trucks so that is just a guess based on some reading I've done.

hope this helps,

Neal

 

You might read this and maybe some help from it. Possible its a hydro boost system issue.

Hydro-Boost Power Assist Systems: Operation Diagnosis and Repair


I had a 2004 ford expedition my sister wanted sold but the brake pedal went to the floor when pressed to stop. Being a girl, she said it felt fine to her.

I had front brakes put on but that did not help the soft pedal. Another mechanic replaced the master cylinder and tells me its all fixed. Nope, pedal still soft to the floor.

Looking at the internet cause it has all the answers in the universe, I see the ABS unit has to be bled. Ford has a scan tool to actuate each of the four abs valves but it still can be down by a mechanic opening the bleed valve and bleeding the abs.

After the other mechanic bled the abs it worked fine.

 

Its the Hydro boost. they are a pain in the *** but those are the symptoms. I've changed a few on different chevy products over the years.

 

Thank you boys for your help,

I know the hydro boost is quite sensible part but until now, It looks fine : to normal noise, pedal return is ok, no self applying brake...
Tell me if I'm wrong but there is no hydraulic connection between the booster and the brake lines, so I don't understand where does air is comming from ?

What do you think about ?

 

Hi legeren, Thank you for your really interesting feedback.

It could be the cause of the problem because when I apply the brake, I can feel the disk runout. Not in the pedal but in the behavior of the car when it slow down.

I'm not the first owner (It's a second hand truck imported from california). I run for 60.000 miles with, and I always had the "runout" behavior when applying brakes. I don't know if it a normal behavior with the H2 or not....

Important remark : I have seen on the rear axel, the braking pads doesn't touch the disk on the whole surface. I noticed this is only with the both inner pads, the both outside pads are ok....... Does it mean something wrong with the caliper ?

Do you think if air comes in at the rear axel, it could goes up to the ABS block and spread on the 4 brake lines ?

I m gonna take a picture.

Thank you for your precious help !

 

Do you have a place that can resurface the disc rotors for you? If the rotors are warped and the pads are not in great condition I would start with those.

 

That's a hard question to answer, the front and back braking systems are separate. The only spots I know of that they are close together or isolated with O-rings are the master cylinder, ABS block and the proportioning valve if it has one (I have not looked to see).
The air could possibly migrate to the high point of the system which would be the master cylinder and the ABS module but I don't think it will cross from back to front without other issues.
The only thing that could explain air in front and back is if you have more than one rotor with runout issues.

Neal