BRAKES losing presure! any ideas

[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.

[sbsisco]

no offense at all...actually with my trip planned to colorado next month and a known break issue I can't figure out ..has got me motivated to experiment with that very thing - just incase. I'm pretty shure it would be a really low chance I'd completely loose front and rear parts of my master cyl in the same trip but it could happen. I'm taking it in to another dealer in the morning and hopefully with the explanations its going to the san juans they'll look a little closer than the others have...at the very least I'll keep my reciept to prove i tried to get it fixed incase those brakes force me to take the fast way down black bear pass..seriously, with it in 4 wheel low and the fact the pedal has yet to go all the way to the floor (ebrake as back up plan)...I'm pretty certain I can keep it on the trail but I'd feel a lot better if I can get that MS changed out. At least it won't take me by suprise, its been doing it long enough im getting used to it.

[Big Dad]

I hope you don't take this $hit the wrong way...I'd hate to hear that someone crashed while using the ebrake....

You do know that while you're driving it's best to pull the brake release handle/ lever (depending on the application) while you apply foot pressure to the ebrake, that way if you start to lock up the brakes you can ease up on the ebrake and it'll retract to it's normal position instead of locking up the brakes.

Again, sorry to imply the obvious but a lot of people don't know how to really brake using the ebrake.

As far as the problem goes, I haven't noticed anything like it with my 3. I have about 22k on it at this point. I'm sure you'll figure it out though.