Has anyone had this problem and/or have any suggestions for a fix. My 06 H3 (25k miles) looses break pedal under light breaking but not under normal to strong breaking. It normally has excellent pedal but every now and than under light presure it will quickly drop half way to the floor..? I've just recently discovered that at highway speed I can get it to do this by lightly holding the pedal with my left foot and with in a small amount of time it'll drop, if i ease up just a little and reapply it'll drop a little further and if I stay at it I can work the pedal all the way to the floor. No warning messages, no leaks, plenty of fluid and excellent pads...Of corse it won't do it when its returned to the dealer so I've gotta trouble shoot this one on my own due to the lack of quality shops/mechanics in my area to trust working on my truck. I'm plannig a trip to sw colorado next month and bad breaks on black bear pass are not a good combo...any ideas, my next move is to bleed the system in the morning just to see if it could be something that simple...

 

Sounds like a brake fluid/ hydraulic issue.

Get that checked out at the dealer, sounds serious

 

I was going to mention bleeding them. Other than that definitely would take it somewhere before the trip

 

Ya, bleeding had no effect on the issue. as well, the only hummer dealer around says I don't have a problem b/c it won't do it when they drive it...and after hours online I can not find any online parts dealer who has a master cyliner for an h3 or a colorado truck..plenty of articles online about colorados having this issue and a few parts places that sell master cylinder caps w/ the same part number for h3/colorado so I'm guessing they share the same MS...on the positive side...i have gap ins coverage just incase she decides not to stop....

 

Any signs of leakage at the calipers? ( brake fluid on the wheel or tire ) Had any work done on the truck? What is the brake fluid level like? Has anyone added any fluid? Sounds like you are losing a little fluid somewhere. ( bad diaphragm) There was a thread on this not too long ago but I think it was in the H1 section. Problem sounds a lot like yours and I think it turned out to be the master cylinder. Do a search for that and something should come up. When bleeding the brakes I am sure you didn't let the brake fluid level get low enough to get any air in the lines.

 

At leasat you have a positive attitude about it..... don't forget to buckle up.