Booster bad? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

demian5 · 10-27-2016, 02:17 PM

My truck runs like I have a vacuum leak. The booster feels like it has some sort of boost, however when I pinch off the booster vacuum hose, the truck idles down a bit and the brakes feel like manual and require a ton of foot pressure to operate.

Right now, the truck idle has to be set to a point where the air fuel mixture screws don't effect the idle quality (it actually sounds weird at idle like its struggling to run off the idle mixture screws, almost like a puff puff sound). Off idle, perfect runner. No surging or anything. The only other vacuum line is going to the trans, and I pinched that off with no change in idle.

I just replaced the intake manifold gaskets because the exhaust crossover areas were blown out and it had an exhaust leak at the intake (that's a new one for me). It idles the same before and after, just quieter now...

Its basically a stock 350 with a turbo 350.
Has an edelbrock Q-Jet Carb that I pulled apart and cleaned it up (wasn't dirty).
MSD billet distributor with MSD blaster coil (no ignition box) set to 10+ BTDC in park at 700 RPM. I removed the vacuum advance hose and capped all the vacuum ports off.

Maybe someone has some insight. Thanks.

**kwmech** · 10-27-2016, 02:51 PM

Run the engine for a bit, then shut it off. Pry the hose and check valve off the booster can and see if there is residual vacuum. If not, booster needs replacement. Also, pinch off the hose to the booster and see if the engine changes the way it runs.. funny thing I just ordered a new booster and MC for my truck just this morning. For me the brakes worked fine, but the engine started running a little weird at idle.

geezer#99 · 10-27-2016, 03:02 PM

Put a vacuum gauge on it.
You likely don't have the needed vacuum to operate the booster.

snipescastle2 · 10-28-2016, 02:20 PM

I had the same problem, had diaphragm in the. Booster. Changed it out, no more issues.
Ben

demian5 · 10-28-2016, 02:29 PM

Quote:

Originally Posted by snipescastle2

I had the same problem, had diaphragm in the. Booster. Changed it out, no more issues.
Ben

Right on. Thanks.

**davischevy** · 10-28-2016, 10:46 PM

This doesn't address any other leaks, but, to check the brake booster, Pull the hose off the check valve at the booster, and pull a vacuum on the booster. The booster should hold steady vacuum. I usually pump it to about 20 lbs.

I have had a terrible time with leaking brake boosters the last couple of years.

snipescastle2 · 10-28-2016, 11:40 PM

My post was supposed to say " The booster had a bad diaphragm " but I'm guessing you figured that out....lol. At first, I thought the brakes weren't working because of the lumpy cam I'm running (11 inches of vacuum at idle) turns out the booster was leaking so bad it didn't matter how much vacuum I put to it, it wasn't going to work. Changed out the booster and added an electric Vacuum pump (that's how I found out my booster was bad, the electric pump makes 25" of Vacuum, and it should have held it at 25", but it kept leaking down) at least it was affordable to buy a new one.
Have a good one!
Ben

10-27-2016, 02:17 PM	#1
demian5 Registered User Join Date: Aug 2016 Location: Lake Forest, CA Posts: 2,177	Booster bad? My truck runs like I have a vacuum leak. The booster feels like it has some sort of boost, however when I pinch off the booster vacuum hose, the truck idles down a bit and the brakes feel like manual and require a ton of foot pressure to operate. Right now, the truck idle has to be set to a point where the air fuel mixture screws don't effect the idle quality (it actually sounds weird at idle like its struggling to run off the idle mixture screws, almost like a puff puff sound). Off idle, perfect runner. No surging or anything. The only other vacuum line is going to the trans, and I pinched that off with no change in idle. I just replaced the intake manifold gaskets because the exhaust crossover areas were blown out and it had an exhaust leak at the intake (that's a new one for me). It idles the same before and after, just quieter now... Its basically a stock 350 with a turbo 350. Has an edelbrock Q-Jet Carb that I pulled apart and cleaned it up (wasn't dirty). MSD billet distributor with MSD blaster coil (no ignition box) set to 10+ BTDC in park at 700 RPM. I removed the vacuum advance hose and capped all the vacuum ports off. Maybe someone has some insight. Thanks. __________________ "Work hard, use your vacation days." 1970 C15 GMC Long Bed 1986 C20 Scottsdale 1983 K2500 Sierra Classic Suburban 6.2 Instagram: C10sofOC

10-27-2016, 02:51 PM	#2
kwmech Senior Member Join Date: Mar 2003 Location: Colfax-California Posts: 8,600	Re: Booster bad? Run the engine for a bit, then shut it off. Pry the hose and check valve off the booster can and see if there is residual vacuum. If not, booster needs replacement. Also, pinch off the hose to the booster and see if the engine changes the way it runs.. funny thing I just ordered a new booster and MC for my truck just this morning. For me the brakes worked fine, but the engine started running a little weird at idle.

10-27-2016, 03:02 PM	#3
geezer#99 Registered User Join Date: Aug 2009 Location: Bowser Posts: 13,712	Re: Booster bad? Put a vacuum gauge on it. You likely don't have the needed vacuum to operate the booster.

10-28-2016, 02:20 PM	#4
snipescastle2 Registered User Join Date: Mar 2009 Location: WARRENSBURG,MISSOURI,64093 Posts: 1,518	Re: Booster bad? I had the same problem, had diaphragm in the. Booster. Changed it out, no more issues. Ben __________________ "Persistence is the twin sister of Excellence.one is a matter of quality,the other, a matter of time" “Serious Black” my 1971 Short Bed Fleetside Sold to an outstanding man in Lees Summit,Mo. “Gone, but by no means,Forgotten”

10-28-2016, 10:46 PM	#6
davischevy Senior Member Join Date: Sep 2008 Location: Harrison, Arkansas Posts: 9,836	Re: Booster bad? This doesn't address any other leaks, but, to check the brake booster, Pull the hose off the check valve at the booster, and pull a vacuum on the booster. The booster should hold steady vacuum. I usually pump it to about 20 lbs. I have had a terrible time with leaking brake boosters the last couple of years. __________________ other Larry Build thread, Arkansas K10 https://67-72chevytrucks.com/vboard/...=755797&page=5 The ability to speak several languages is an asset, but the ability to keep your mouth shut in any language is priceless.

10-28-2016, 11:40 PM	#7
snipescastle2 Registered User Join Date: Mar 2009 Location: WARRENSBURG,MISSOURI,64093 Posts: 1,518	Re: Booster bad? My post was supposed to say " The booster had a bad diaphragm " but I'm guessing you figured that out....lol. At first, I thought the brakes weren't working because of the lumpy cam I'm running (11 inches of vacuum at idle) turns out the booster was leaking so bad it didn't matter how much vacuum I put to it, it wasn't going to work. Changed out the booster and added an electric Vacuum pump (that's how I found out my booster was bad, the electric pump makes 25" of Vacuum, and it should have held it at 25", but it kept leaking down) at least it was affordable to buy a new one. Have a good one! Ben __________________ "Persistence is the twin sister of Excellence.one is a matter of quality,the other, a matter of time" “Serious Black” my 1971 Short Bed Fleetside Sold to an outstanding man in Lees Summit,Mo. “Gone, but by no means,Forgotten”



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