Vacuum booster

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   I'm at my wits end with this customers 1999 chevy silverado pickup.  Comes in with a broken brake line. Of course gm had to put the abs module under the can so you have a bundle of lines running along the frame rail and that's where it's leaking.  Put in new lines and bled it.  Took it for a drive and the pedal was hard with little braking effort.  

Bring it back in.  All the calipers move nicely and I cleaned up the pads so they float.  

Took it out again same story.  Doing some investigation I find that I have next to no fluid coming from the abs to one of the front wheels. Seen this before abs fails. Got a junk yard abs and now I have good fluid flow to all wheels using my pressure bleeder.  

Drive it again and same thing.  I've run out of things to check except for the master and the vacuum booster.  

Pull the vacuum line and the booster woosh's. So I know it holds air.  Start pickup with line off and you have good vacuum. 

My question is can I have a booster that's junk but yet still holds vacuum?  What are your guys thoughts

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bobkyllo Members Profile Send Private Message Find Members Posts Add to Buddy List Orange Level Joined: 14 Sep 2009 Location: minnesota Points: 1568	Post Options Post Reply Quote bobkyllo Report Post Thanks(0) Quote Reply Topic: Vacuum booster Posted: 29 Mar 2026 at 5:22pm
	I'm at my wits end with this customers 1999 chevy silverado pickup. Comes in with a broken brake line. Of course gm had to put the abs module under the can so you have a bundle of lines running along the frame rail and that's where it's leaking. Put in new lines and bled it. Took it for a drive and the pedal was hard with little braking effort. Bring it back in. All the calipers move nicely and I cleaned up the pads so they float. Took it out again same story. Doing some investigation I find that I have next to no fluid coming from the abs to one of the front wheels. Seen this before abs fails. Got a junk yard abs and now I have good fluid flow to all wheels using my pressure bleeder. Drive it again and same thing. I've run out of things to check except for the master and the vacuum booster. Pull the vacuum line and the booster woosh's. So I know it holds air. Start pickup with line off and you have good vacuum. My question is can I have a booster that's junk but yet still holds vacuum? What are your guys thoughts

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DMiller Members Profile Send Private Message Find Members Posts Add to Buddy List Orange Level Access Joined: 14 Sep 2009 Location: Hermann, Mo Points: 35650	Post Options Post Reply Quote DMiller Report Post Thanks(0) Quote Reply Posted: 29 Mar 2026 at 5:25pm
	You have a Bad booster chamber. Is not applying the stored vacuum. Inner works are screwed.

bobkyllo Members Profile Send Private Message Find Members Posts Add to Buddy List Orange Level Joined: 14 Sep 2009 Location: minnesota Points: 1568	Post Options Post Reply Quote bobkyllo Report Post Thanks(0) Quote Reply Posted: 29 Mar 2026 at 5:49pm
	I was leaning to the booster but I was unsure if you could have a bad booster but yet have no leak or poor driveability. All the bad boosters I've ever come across would not hold vacuum. Thank you for the reassurance.

DMiller Members Profile Send Private Message Find Members Posts Add to Buddy List Orange Level Access Joined: 14 Sep 2009 Location: Hermann, Mo Points: 35650	Post Options Post Reply Quote DMiller Report Post Thanks(0) Quote Reply Posted: 29 Mar 2026 at 6:23pm
	Can fail either way.

Ed (Ont) Members Profile Send Private Message Find Members Posts Add to Buddy List Orange Level Joined: 08 Nov 2009 Location: New Lowell, Ont Points: 1590	Post Options Post Reply Quote Ed (Ont) Report Post Thanks(0) Quote Reply Posted: 29 Mar 2026 at 7:12pm
	I find on a lot of this newer stuff you have to hook up computer and activate brakes with it. I had a 99 GMC2500. Did all the brake lines at once due to rust issues - plow truck. Could not get a pedal at all even after lots of manual bleeding. Finally hooked up computer, activated ABS a couple times. Quick manual bleed and 100% brake operation!!!! Drove it for a few years after that with no issues and great brakes. Finally sold the truck and it is still working today. New owner turned it into a tire service truck and loves it! Edited by Ed (Ont) - 29 Mar 2026 at 7:16pm

DaveKamp Members Profile Send Private Message Find Members Posts Add to Buddy List Orange Level Access Joined: 12 Apr 2010 Location: LeClaire, Ia Points: 6119	Post Options Post Reply Quote DaveKamp Report Post Thanks(0) Quote Reply Posted: 8 hours 43 minutes ago at 1:47am
	I don't like to hear it, but I"m not surprised you're experincing this situation, Bob- I had a '99 Silv that darned near killed me when one brake line broke, and the ABS system prompty emptied ALL the brake fluid out onto the ground. I had a trailer in tow, with about 2600lbs of lumber stacked on it... I was going 35mph, on flat land, approaching an intersection where crossing traffic did not stop... and there was a 5-axle dumptruck coming from my left, and TWO loaded semis coming from my right. I must've been a little nervous, because I broke the E-brake cable right off when I stepped on it. Fortunately, quick thinking I grabbed the trailer electric independant, and locked them up... all the crossing drivers saw the smoke as I was rolling and realized what'd happened... they couldn't stop, but they moved far enough to keep me alive... I finally got stopped about 3ft into the intersection, but AFTER they'd moved far enough to squeak by me. GM really failed here- the ABS control system scheme should NOT have allowed that, and they should NOT have used such lousy materials in the brake lines. They used same steel tubing in fuel lines and vent line, too, all terrible with respect to corrosion resistance. That situation meant that NO shop would do anything less than a complete and full replacement of EVERY brake component. After finding that the ABS pump and plumbing was wedged in where it was, I cut the engine and transmission out (the 4L60E was of the 'problem' series that required a $4200 rebuild after ring gear broke 8 months before). They're lemons, so I made lemonade. I removed and sold the engine, trans, and T-case, cut the frame, cab, and body into pieces, and took it to the recycling yard, to make certain it would NEVER hurt ANYONE after that.
	Ten Amendments, Ten Commandments, and one Golden Rule solve most every problem. Citrus hand-cleaner with Pumice does the rest.