GM's hydro boost

[cybrfire]

Is it possible for there to be an issue with the hydro boost that would apply the brakes? An internal leak of some sort that would push the master cylinder without you having touched the brake pedal itself?

My crew cab brakes are applying themselves without any input on the brake pedal.

It gets to the point where you have to start dropping gears. When you finally give up and pull off the side of the road, the brake pedal travels much further than is normal. Hubs are hotter than they should be on both sides, front axle only.

This happens while you are at cruising speed with no pedal input. They simply start grabbing.

Everything else turns freely. Jack up the front end and the tires won't turn. Give it some time and they free up again and roll smoothly.

 

[cybrfire]

just found this:

[FONT=cd9800348d99a6a1114ac7a0#dc1700]5. [/FONT]If brakes are self applying and pedal is free, check for obstruction in the return line or a kinkedconnection between hydo-boost and pump reservoir. If obstruction or kink is found, go to step6, otherwise go to step 7.6. Remove obstruction or replace line as required. If condition remains, check for a damagedreaction end. If damaged the hydo-boost should be replaced or repaired.

http://www.scribd.com/doc/6829161/Operation-Diagnosis-and-Repair-of-HYDROBOOST

 

[thearborbarber]

Pretty cool link. I just had to replace my hydroboost because of a leak. It's a pretty nice and simple system (at least when it works).

 

[diesel4me]

I dont know much about the hydroboost ,but if there is not enough free play in the push rod to master cylinder connection,the brakes will start dragging after the fluid heats up some and it will apply enough pressure to the master to keep the brakes applied,it heats up more,then worsens..when it cools off,they release..since the frront brakes usually are calibrated to apply before the rears slightly,this might be why your front ones seem to be doing it the most..

Next time it seems to be dragging,get out and loosen up the two master cylinder bolts some,then see if it'll roll freely...if it does,its not enough free play where it needs some at the push rod..

My 82 k2500 seems to have draggy brakes lately too,but I think its the RF caliper,only that wheel heats up..going to put another rebuilt on it and replace my master cylinder too,I'm tired of having to fill the rear chamber up daily,it leaks out of the seal where it bolts up--spent more on brake fluid than a rebuilt master costs twice by now I bet..

 

[cybrfire]

I dont know much about the hydroboost ,but if there is not enough free play in the push rod to master cylinder connection,the brakes will start dragging after the fluid heats up some and it will apply enough pressure to the master to keep the brakes applied,it heats up more,then worsens..when it cools off,they release..since the frront brakes usually are calibrated to apply before the rears slightly,this might be why your front ones seem to be doing it the most..

Next time it seems to be dragging,get out and loosen up the two master cylinder bolts some,then see if it'll roll freely...if it does,its not enough free play where it needs some at the push rod..

My 82 k2500 seems to have draggy brakes lately too,but I think its the RF caliper,only that wheel heats up..going to put another rebuilt on it and replace my master cylinder too,I'm tired of having to fill the rear chamber up daily,it leaks out of the seal where it bolts up--spent more on brake fluid than a rebuilt master costs twice by now I bet..

I'll try the 2 bolt deal as a test once I visually inspect the return lines. Symptoms are exactly what you describe though.

We have been working on the AC system lately. That belt just happens to run the powersteering pump as well so there may be some cross mojination going on here. It's possible I adjusted the tensioner and kinked a line without realizing it.

 

[76zimmer]

cross mojination...like that one!
I agree with the kinked line...seems like it gets warmed up, maybe expands some, and won't allow the fluid to flow...just like an old brake line...pressure builds up and can't be released causing the brakes to apply....got a infra red temp gun? Like a welder, so many uses for them once you have one!

 

[diesel4me]

I forget which vehicle I ran into this on,but it was the lack of free play..I changed a master cylinder and there was a plastic spacer thing like a gasket between it and the power booster,and it ended up sticking to the old master cylinder and sent back with the core!..so after I figured out why the brakes eventually locked up after some distance,I ended up having to hunt one of those things down at a junkyard...the push rod in that vehicle was not adjustible like some were..

What is wrong with yours could be something different,I find it not too likely that the free play would change suddenly ,unless you had just replaced the master cylinder or something...it could be in the hydroboost itself I suppose..

 

[cybrfire]

The master is fairly new. I've been driving on it for a few months. This is a fairly new problem.

After 10 or so miles the front brakes start to drag. At first I thought the calipers were not releasing but after it happened a few more times, I realized that it would happen regardless of whether you had used the brake pedal.

If you let it sit for 10-15 minutes it goes away and all is back to normal. You pretty much have to realize the problem is happening and immediately get it to a spot where you can jack up the front end and roll the front tires. Only other indication is the heat in the hubs and rock guards and a hot odor.

EDIT: I should also note, the brakes are awesome. I've never had a pedal so firm in anything i've driven. Maybe that's the problem. When you first start the truck there is a touch of freeplay. After driving for a little while, there is almost none.