Weird hydroboost issue.

Astro

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Hey all,

So I have a really specific issue with my HB. Just to preface, my 96 sierra k1500 used to have a vacuum power assist. I purchased a HB from a junkyard out of an 03 Suburban, rebuilt it (orings and seals) and installed it onto my truck with all new lines.

I went through the process of bleeding the power steering. I had to use an old school MC so that I wouldn't have to change the brake line fittings. It's one of the those all iron ones. All I had to do was switch front and back lines on the MC to get it to work. Also, I had to space the MC about 1/16 of an inch away from the HB so that the pushrod wouldn't always be depressing the MC piston.

Ok so here's the issue, when I apply the brakes, it stops on a dime. Feels great. But when I let go, it stays engaged for a second or two, then disengaes slowly on its own and returns fully back. There's always a delay though. I let go of the pedal and it's like, "deh maybe I should let go? OK I'll let go". I have tried to return it faster manually with the top of my foot, but it actually resists me. It insists on returning at its own pace.

Anyone have any ideas as to why this might happen?

Thanks so much for any and all ideas!
 

Schurkey

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Perhaps a moderator will move this from the "Engine" section to the Axles + Brakes section.

1. Do you have low-drag calipers on that 1500, like almost all of them did? If so, you have the wrong master cylinder. Or you have incompatible calipers that need to be changed. Or both.
2. What master cylinder bore size did your truck have originally? What master cylinder bore size do you have now?
3. Did you change to the correct brake pedal for Hydroboost?
4. HOW did you space the master cylinder "forward" on the booster? Are you sure the primary piston isn't being forced just a little bit forward, restricting the cylinder-to-reservoir ports?

Overall, it sounds to me like you need to ditch that master cylinder for one that actually fits and works properly; and you'll probably have to change calipers, too.

If you have the shiity 254mm (10 inch leading/trailing shoe) drums in the rear, you need to think about upgrading them, next.
 

RichLo

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^ Agree'd

Most people who upgrade to a HB on their half ton do it from the parts store counter instead of the JemYard. If you match 1996 3500 parts to your 1996 1500 everything will function just like a proper 3500 should.

Also, to Shurkeys point above, what year do you consider 'old school' for your master cyl?
 

someotherguy

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All I had to do was switch front and back lines on the MC to get it to work. Also, I had to space the MC about 1/16 of an inch away from the HB so that the pushrod wouldn't always be depressing the MC piston.

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Astro

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Honestly, I do not know what calipers I have other than that they are stock. It's a special trailering z71 package. So it's a 6 lug with a 14 bolt rear end and some different stuff on the suspension. I'm not sure if the brakes were different between the regular z and mine.

It does have drum brakes, but I'm not sure what size. I DO need to get into them soon though, so I'll check. What's wrong with those brakes? I DO prefer disk of course!

The MC is the full iron type that came in like 70's and 80's cars. The spacing was only 1/16 of an inch. Very small. After speaking to Sweeting performance about it, they told me I probably didn't need to do that. The only reason I did was because I had a custom setup on my 66 Lemans years ago and had trouble with the brakes dragging. It turned out that mc and booster combo wasn't correct and it was depressing the MC piston. Spacing it fixed the problem. The way I did it was to put two washers, on either stud, between the MC and HB. BTW I got all these parts from them, less the HB. They told me this was a working setup and explained to me that I would have to switch ports on the MC.

So the pedal is the same. There isn't any variation in how the rod engages with the pedal though (compared to my vac booster). The only thing I had to do was get an adjustable rod to lengthen it. Otherwise it comes out straight and mounts straight. Is there something I missed there?

So I bled the system again because I discovered that I didn't do it correctly the first time and the problem lessened. I may go through the process again to see if it gets even better. I wasn't aware of the added step to depress the brake pedal 3 times between cycles of rotating the steering wheel.
 

Schurkey

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Look in the glovebox, at the SPID (Service Parts ID) sticker. Wild Guess: You had JB6 brakes, which means low-drag front calipers and the special, matching Quick Take-Up "three chamber" master cylinder.

Those calipers NEED the third, high-volume, low-pressure chamber to work properly. If the Quick Take-Up master cylinder won't fit the Hydroboost, I guess you need the correct Hydroboost master and the associated Hydroboost calipers.

But that still leaves the brake pedal which is different between vacuum booster and hydroboost. Different geometry for the pedal pushrod. Or something.

But on the brighter side, it sounds like you probably have the 11.13-inch rear drums (JB6) instead of the crappy 254mm (10 inch) drums (JB5.) Ordinary rear discs are not inherently better than the good 11.x Duo-Servo drums. I don't doubt that the aftermarket makes bigass rear disc conversion kits...but the bottom-feeder "budget" kits are a disaster.
 
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Supercharged111

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I believe the bore of the HB from an 03 would have a smaller ID, so you'd have to waller out that or buzz down the original master to make that fit. And of course space it correctly.
 
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