BRAKES losing presure! any ideas

[sbsisco]

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 got to 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...but i have a hard time believing this will help but i guess it could have left the factory with air in the lines???

[wpage]

From what you describe it sounds like air in the line.

[Yetti]

Ok lets start slow.

1. if you pull up on the pedal with your toe does it make any clicking noise? if so the pedal might have been held down by the brake light switch. it can and will prevent the pedal from coming to the top and letting the master cylinder refill after a brake apply.

2. if you ease the pedal down like you discribed is there a point where you feel the brakes pick up?
I suspect if the brake light switch issue doesn't fix the problem then I would put in a new master cylinder. the reason being the master applies the rear brakes first. then as it picks up load it applies the fronts. that way the systems has the default of not letting the fronts lock up first and causing an accident. I would say the master has worn out just enough to vent pressure around the seal on the apply piston under light load. if you hit it fast it applies normal because the seal is forced to stretch to meet the bore imperfections.

[sbsisco]

My thoughts as well on the master cylinder. thats my next investment if todays bleeding doesn't help. I've had a lot of master cylinders go out on classic cars I've owned and my way of testing for that was to sit stopped while pushing hard on the pedal and waite to see if it'll slowly start to sink to the floor. This won't do that..yet,,,it may later on if its is an o ring in the MC. My main reason for posting is to see if anyone has had this issue and what it ended up being. I only have my years of mechanical experience on the old cars without all the extras to the brake system. Tearing into a new hummers brake system seams a little scary to me if it could be caused by some kind of sensor or eletronic issue I don't understand...all comments or suggestions are greatly appriciated...thanks...

[sbsisco]

Bleeding helped but still has the symptoms. not the break light switch, although that was a great suggestion that I haven't thought of. It does find a stopping point about mid pedal so I'm sure your right, its a rear brake issue with the master cylinder....did discover the hard way that it has to be running to bleed the brakes. with it shut off no amount of pumping with push fluid to the calipers....with it running we bled thru each wheel 6-7 times and never really got any air in our fluid recovery line. As supprising as this is, I can find countless h2 master cylinders cheap online but nothing for h3 or colorado trucks. plenty of forums complaining of this issue on colorad truck sites I found tonight. not sure if they share the exact same master cylinder but i did find the caps have the same part no. and thats as far as i got today. the only dealer near me has seen it twice and says nothings wrong so until it has a total failure I guess warranty is out of the question......hmmm..

[Big Dad]

This is probably going to sound smartassy but it's not intended to be.

Have you ever used your e-brake?

Till you get this anomaly figured out I'd suggest that you actually use the e-brake a few times, you know, in case you ever really need to you'll know how it reacts.

A lot of people just take the e-brake for granted and never use it or never really know how to use it when the immediate need arises.