Hydro-boost issues Q?

[special-K]

I've had quite a few trucks with hydro-boost and never had any issues before. I'm not a professional mechanic and have no understanding of the inner workings through experience. That time is now!

The issue is,sometimes I have nothing...pedal goes to the floor! I let the pedal return and I may get pedal or I have to give a couple/few tries. The rest of the time the pedal is low. I put a known to be good master cyl from one of my trucks and that made no difference. My instincts tell me there are internal seals that must be bad. It seems worse in cold temps and mostly when I first start it up. What do the guys with such wisdom think? And,what would be my best way to go...repair or replace? I have no idea about parts cost,how complex,reliable it would be to repair or if special tools are necessary. Don't have the Helm book ('85 6.2 truck) or GM parts book,so no exploded views. It appears to be pretty simple though. Could someone educate me?

[Oldriginal86]

Looked in the 87 service manual. No symptom of excessive pedal travel in the hydro boost section. Only ones they listed were as follows:
1. Slow pedal return
2. Grabby brakes
3. Accumulator leak-down
These didn't sound like your issue. Looked in brake section under excessive pedal travel.
These possible causes were listed:
1. Low fluid level in master cylinder.
2. Air in brake system.
3. Malfunctioning self adjusters.
4. Bad master cylinder.
5. Incorrect wheel bearing adjustment.
6. Improperly adjusted M/C pushrod.
Hope this helps.

[JellyBeanDriver]

I don't think the hydroboost can give you those symptoms.

Per this picture, the input shaft goes right to the output shaft.

I would say you have a bad MC but you said you swapped in a good one with same problems.

Could there be something wrong with the rest of your brakes? Like some clearances being taken up? (bad adjuster) or excessive runout/ bad wheel bearing in the front so the rotor is pushing your caliper piston back in? Our Dually, the brakes would not grab for a portion of the pedal press but this was due to misadjusted front wheel bearings.

Or is there a brake proportioning valve that can cause this?

[old Rusty C10]

go to the steel soldiers website this is probably a military type system prolly has the part numbers too

[Rubble]

I just went thru something similar on my 81 K35 crew SRW.Sometimes the pedal was firm,others almost go to the floor.I just got done swapping in a newM/C & made a world of difference.Firm pedal now & rear brakes work good now!! I don't know if this is same as your problem,but thought I would throw it out there anyway eh...Good luck

[special-K]

Thanks everyone. I'm stumped. I figured it was my lack if understanding of the system keeping me from figuring this out. It just had a really low pedal when I bought it,and brake light was on. It's a 1/2t diesel. I drove it home about 5 miles and not again till after the axle swap. I did nothing because the first thing planned was swap-in 8-lug axles. Did that and still the same. The brakes bled out ok at that time,but the brake light was still on and pedal low.The axles were from the same vehicle the master cylinder came from,which I drove for a few years before the engine went up. Along with the axles came fairly fresh braided stainless brake hoses. The master cyl I did after the low pedal didn't go away. That changed nothing. After more driving is when the pedal to the floor/no brakes thing started. It seems to be mostly when I first start driving,but not 100% predictable. I stopped trying to drive it,but while I was trying to figure it out I'd try the brakes early just in case and the second or third time they'd be there (if not the first. Pretty scary and no good for emergency stops! I just figured it was the hydroboost because other than the hard lines and prop valve everything is from another vehicle that had great brakes. Not losing any fluid at all
I'm wondering about the prop valve. Never had one act like this, Never had anything act like this. I'll start it up,check the brakes and they're there. Put it in gear,let it roll,hit the pedal right away,and more times than not the pedal goes to the floor with no brake at all. I'll let off right away,wait for it to come up (slowly),try again and maybe yes maybe no. By then I use the parking brake because my drive starts downhill

[JellyBeanDriver]

Forgot to post the picture. Here's a cutaway of a hydroboost setup.


Input shaft is straight connected to the output shaft to the MC.

[special-K]

hey thanks! Something to study and ponder on. Funny how you can drive a vehicle with such a component for so many years and be unfamiliar with it just because you never had to deal with it. My '92 I've had since new,has 300k on it,and all I know is it stops

[special-K]

So,no one has ever seen a problem like I describe? I just figured it was the hydroboost by the parts already replaced,but I'm open to checking something else. It wouldn't surprise me if I'm having a problem no one has ever seen. Wouldn't be the first time. Usually when someone tells me "I've never seen those never go bad" that's what it ends up being

[D13]

Your problem has nothing to do with hydroboost. Hydroboost either gives you assist or it doesn't. Pedal to the floor is usually M/C, either seals or low fluid due to leaks or air bubbles working themselves out. Mu 99 did exactly like yours, found brake lines rusted, would hold pressure so no visible leaks, but would suck air in. Finally figured the issues out when it peed all over the bottom of the truck. Since you did an axle swap, suspect the connection at the rear hose, or an air bubble.

[DirtyLarry]

Originally Posted by D13

Your problem has nothing to do with hydroboost. Hydroboost either gives you assist or it doesn't.

I agree! Pedal to the floor is indicative of a master cylinder issue or a loss of hydraulic pressure somewhere (leak). If there was a leak Tim would have found it by now....this has to be a m/c issue. Hydroboost will not cause an issue like that. Either you have breaking power or you don't, just like when a vacuum booster fails or loses its vacuum source.

[special-K]

I think you guys are right,and that's where my thoughts have gone since I last posted. I was going by the parts that were known to be good and it left the hydroboost,which I never had to service on anything. I was barking up the wrong tree! And,I never had a master cyl work some,not some,and not lose fluid somewhere. I've driven it all day and had brakes,just not proper pedal. But every known to be good part goes bad at some point. And,I've looked into how the hydoboost works and it would have no affect on this. When I looked at this diagram and saw accumulator I wondered about that. I learned it just stores pressure so you have it when vehicle is first started...before it can build pressure. Today a friend was over and he pumped (slow) while I bled with engine static. First time pedal down I got nothing at each corner,then a dribble,and 10x made no difference. I think maybe when bleeding the used master cyl I installed,it went past it's travel limit it and the seal rolled hitting a ridge in the bore. It may seal some and sometimes not,but very little pressure. That's why little or none. That's a theory. Also it sat idle for some time and it could be it pitted. It's just not making pressure. After all that pedal pumping we cracked a line off the master cyl and got what I wouldn't call a squirt. It's weird because it bled when I installed it. Yeah,it's got to be the master cylinder. Thanks for the help. Out of all this I learned how the hydroboost works and another way a master cyl can act when it's bad.

[dwcsr]

I agree with the hydra boost no being in play here. I have seen this when the rear brakes are way out of adjustment and/or a bad front wheel bearing. I would tighten up bearings and rear brakes and then go after the master.

[special-K]

Thanks for your offering. Checked into the issue yesterday and eliminated both of those possibilities before trying to bleed. It's looking like the master cylinder (99.9% sure)

[79Betty]

It's ok i have the opposite problem. Mine stay on until i shut the motor off. :/

[hrc200x]

A co worker of mine with a mid 90s ford had a similar issue, I told him to replace the master cylinder because I seen that similar problem in a 80s chevy, somtimes brakes, other times no brakes and right to the floor, he replaced it and didn't fix it. His problem ended up being the rear brakes out of adjustment or they were seized up. I can't remember which, if it was seized they wern't pushing against the drum they were seized in the pulled away position, truck drove and rolled fine.

[special-K]

You know,I have such suspicions. I'm figuring on inspecting the rear brakes once I get the Master cyl. The rear brakes didn't seem to need adjustment,but if they're froze back they'd seem adjusted up from the outside. Like I said,the Suburban had been sitting a good while. I'm replacing the master cylinder,regardless.