New hydroboost to master cylinder question

[cofferson]

Hi everyone! First post here, first GM vehicle, and first hydroboost vehicle. I have a 2008 Suburban 2500 that I haven't been able to drive much since purchasing because of brake lock up issues. Long story short, I'm at the point of replacing the hydroboost and master cylinder. I purchased a Bosch hydroboost and an AC Delco master cylinder. When I went to install, I learned that hydroboosters aren't adjustable push rod like a typical vacuum booster is. When I push the MC up to the booster, the pushrod bottoms out before the flanges meet. The gap is ~1.4mm which means when I clamp it down with the nuts, it will depress the MC that much. At this point I just don't know what to do with it to make sure I don't have more brake issues when putting this together.

Thanks!

 

[j91z28d1]

I've never messed with a hydro boost on our trucks. but have other stuff. are you sure there's no adjustments anywhere? even on the pedal side?

cause it does really need a way to adjust it. from my experience if you bolt that together with the master dispreased it will cause issues as you drive it.

 

[rdezs]

Double check your part numbers for the application.

 

[rdezs]

A common issue with dragging brakes originates with the OEM flexible brake lines to the calipers. They're rubber, and they start disintegrating on the inside. The pieces end up in the caliper causing what you describe. Aftermarket ones are available that are lined with PTFE on the inside. Or pick up some nice aftermarket stainless steel lines, also lined with PTFE on the inside. And don't forget the two flexible hoses in the rear that go down to the differential.

 

[j91z28d1]

A common issue with dragging brakes originates with the OEM flexible brake lines to the calipers. They're rubber, and they start disintegrating on the inside. The pieces end up in the caliper causing what you describe. Aftermarket ones are available that are lined with PTFE on the inside. Or pick up some nice aftermarket stainless steel lines, also lined with PTFE on the inside. And don't forget the two flexible hoses in the rear that go down to the differential.

Yes, but also I can't 100% tell you if you install a new master with the shaft adjusted a bit to long so that it acts like the brakes are slightly applied. it might not drag for the first few minutes but the more you drive it. they will slowly apply and not release. once left to cool for a few hours they will release and the apply again. I've seen it a few times over the years at work. tow it back, shorted the shaft and alls good.

you don't seem it much in cars because the application is specific, so just bolting on a new master is fine 99% of the time. but my job we have a some old equipment and everything is always different. I've seen it over looked a few times and unit left with locked up brakes an hour after a brake job like wtf?

brakes need some free play at the top of the pedal travel to work. right.

 

[rdezs]

I'd make sure the hydroboost and the master cylinder are both the correct part numbers for the application.

 

[cofferson]

Hey everyone. Just wanted to report back that I finally used an air hammer to separate the original booster from the original master cylinder and they also have the same preload. Anyone here has experience with the hydroboost? I've only ever worked on vacuum boosters and this is a big no no but if the original is this way, who am I to argue?

 

[mikez71]

That maybe why you had the lockup issues in the first place!
I haven't found any evidence that preload is normal...

 

[j91z28d1]

Hey everyone. Just wanted to report back that I finally used an air hammer to separate the original booster from the original master cylinder and they also have the same preload. Anyone here has experience with the hydroboost? I've only ever worked on vacuum boosters and this is a big no no but if the original is this way, who am I to argue?
View attachment 452043

huh. I'd have never guessed. we have a lot of equipment with them, I just changed a master that was leaking out the back a few days ago. never seen one of ours that had preload like that.

 

[j91z28d1]

I don't see anything interesting in my manual for a 08 2500 except this section about bleeding the power steering and pressurizing the accumulator.

other than that it just shows remove 4 bolts and brake switch. nothing noted about adjusting or checking anything.

 

[cofferson]

I don't see anything interesting in my manual for a 08 2500 except this section about bleeding the power steering and pressurizing the accumulator.

other than that it just shows remove 4 bolts and brake switch. nothing noted about adjusting or checking anything.

And there's no adjustment so I'd have to remove the pushrod and trim.. maybe with this amount of preload they designed it to not cover the bypass port. Just seems odd since it's like 1.4mm preload. You have a gmt900 with hydroboost? If you ever get the time I'd love to know if you remove the 2 nuts holding it to the booster, wiggle it back and see if you push back onto the booster if you had a small gap when it contacts the push rod

 

[j91z28d1]

And there's no adjustment so I'd have to remove the pushrod and trim.. maybe with this amount of preload they designed it to not cover the bypass port. Just seems odd since it's like 1.4mm preload. You have a gmt900 with hydroboost? If you ever get the time I'd love to know if you remove the 2 nuts holding it to the booster, wiggle it back and see if you push back onto the booster if you had a small gap when it contacts the push rod

I don't have a 2500, sorry all my hydro boost stuff is on random equipment at work. my gmt900 is one of the hybrid ones. the regen braking system gives me a headache when I start looking into it lol. I might scrap the truck if it ever fails haha.
I just looked up 08 2500 in prodamand before a left work real quick. that's where that pic came from. didn't seem to have much info about it. mostly the vacuum booster trucks.


I would guess you're right thou, if the original was like that and the new ones you got were the same. most likely it's designed that way for who knows what reason. sorry if you shortened your new one and it was supposed to be longer.

 

[cofferson]

It's all good. I used the rod from the original booster in the new one once I got them apart. Doesn't really matter because I have another thread open where the brakes are unbleedable after replacing the master for some reason. Got another ACDelco master cylinder bench bled and on the truck today and still can't get any bubbles at any calipers. Pretty much everything is new now so I'm guessing I have to swap out the ABS valve block which I really didn't want to have to do. I can run the ABS auto bleed but I should have at least got a firm pedal without doing that and then get air out of abs then bleed again..