how do tell if your brake booster is bad? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

modifiedGMC · 09-21-2005, 11:29 PM

That may sound like a dumb question but I just mounted a power brake booster on my 69 GMC (it had manual 4 wheel dsic brakes). I have ran this 4 wheel disc setup with manual brakes for the past year but just wanted to go with power. I got everything mounted hooked up and started bleeding the brakes (engine off). The brakes bled off good so I cranked it up to see how it felt but it didn't feel like power brakes, it was easier than engine off but was still kinda stiff like. I took the booster off of a 86 IROC and lengthened the brake rod going to the pedal. The master cylinder and prop valve are off a 79 caddy that I got the rearend from. It has front and rear disc brakes. I don't know if the booster is bad or something wrong with master cylinder. Is there any way to check booster? I have the vacuum line hooked up right, and there is vacuum going to the booster. Any ideas?

jhow66 · 09-22-2005, 12:26 AM

If your booster is working you will feel the pedal sink when you start the engine .

fastwillie 696969 · 09-22-2005, 01:12 AM

pump your brakes up first hold the brakes down start engine if pedals falls towards floorboard then its normal condition
when its hard to press the pedal down then the booster is shot

Putter · 09-22-2005, 10:20 AM

You know that big Oak tree at he end of the street, right before the boat landing???
Head straight for that...get up some speed....and press the brake pedal. You will know if it is working soon enough!!! (j/k)

72 longhorn BB · 09-22-2005, 10:50 AM

Can't you just use a vacuum checker tool and pump it up to about 20 inches of vacuum and see if it holds vacuum?

Longhorn Man · 09-22-2005, 10:57 AM

Things get trickey when you have a dual daiphram booster...any one have an idea on checking one of them? (bench testing)

sneakysnake · 09-22-2005, 11:23 AM

My experiences with bad boosters, are that most times you can hear a vacuum leak(hissing or sucking sound)or the motor will go into a rough idle when the pedal is pushed due to the tear in the diaphram in the booster.

farside847 · 09-22-2005, 11:36 AM

What is the diameter of the booster? If it is a small one from a car it will not perform as well as the larger truck ones...

modifiedGMC · 09-22-2005, 10:24 PM

JHOW66 & 70BLOWNGMC: the pedal does sink some when the engine starts. I compared it with my wife's IROC and my pedal felt almost but not quite as hers felt before and after cranking the engine (my booster came off an 86 IROC)

PUTTER: so what you are saying if my booster isn't working the oak tree will stop me or the water will soften the stop?

SNEAKYSNAKE: The engine doesn'tidle rough and you can't hear any hissing.

The booster is off an 86 IROC Z28 and the master cylinder is off a 79 cadillac. It's possible the master cyl. could be stiff but not likely.

09-21-2005, 11:29 PM	#1
modifiedGMC Registered User Join Date: Oct 2004 Location: Walker Co. Alabama Posts: 474	how do tell if your brake booster is bad? That may sound like a dumb question but I just mounted a power brake booster on my 69 GMC (it had manual 4 wheel dsic brakes). I have ran this 4 wheel disc setup with manual brakes for the past year but just wanted to go with power. I got everything mounted hooked up and started bleeding the brakes (engine off). The brakes bled off good so I cranked it up to see how it felt but it didn't feel like power brakes, it was easier than engine off but was still kinda stiff like. I took the booster off of a 86 IROC and lengthened the brake rod going to the pedal. The master cylinder and prop valve are off a 79 caddy that I got the rearend from. It has front and rear disc brakes. I don't know if the booster is bad or something wrong with master cylinder. Is there any way to check booster? I have the vacuum line hooked up right, and there is vacuum going to the booster. Any ideas? __________________ If it don't fit, force it,...If it still don't fit , modify it 'till it does fit !!!

09-22-2005, 12:26 AM	#2
jhow66 Registered User Join Date: Jun 2002 Location: Knoxville Tenn. Posts: 3,058	If your booster is working you will feel the pedal sink when you start the engine . __________________ 56 Chevy Bel-Air 2dr. HT (purchased new) 71 Chevy Cheyenne SWB PU (502HO) 65 GMC short bed step--work in progress and my gofer

09-22-2005, 01:12 AM	#3
fastwillie 696969 ~Rest In Peace~ Join Date: May 2005 Location: CALIFORNIA NOR CAL Posts: 9,707	pump your brakes up first hold the brakes down start engine if pedals falls towards floorboard then its normal condition when its hard to press the pedal down then the booster is shot __________________ is it fast ? it has a lighting bolt donut? B___H please, I can remove 90% of your so called "beauty" with a kleenex

09-22-2005, 10:20 AM	#4
Putter More Cowbell.... Join Date: Mar 2004 Location: Puckett, MS Posts: 3,569	You know that big Oak tree at he end of the street, right before the boat landing??? Head straight for that...get up some speed....and press the brake pedal. You will know if it is working soon enough!!! (j/k) __________________ *There once was a member from Puckett.....Who ( fill in the blank blank blank* ) bucket**

09-22-2005, 10:50 AM	#5
72 longhorn BB Registered User Join Date: Jan 2000 Location: Michigan Posts: 312	Vacuum tool? Can't you just use a vacuum checker tool and pump it up to about 20 inches of vacuum and see if it holds vacuum? __________________ Chuck 1972 C-20 Longhorn 402 BB .030" over (409?), curved HEI, RV cam, quadrajet, dual 3" exhaust, TH 400 ported w/shift kit 1999 K1500 Suburban (old style) 5.7L Vortec 4L60E "No matter where you go, ........ there you are"

09-22-2005, 10:57 AM	#6
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,690	Things get trickey when you have a dual daiphram booster...any one have an idea on checking one of them? (bench testing)

09-22-2005, 11:23 AM	#7
sneakysnake It's a catastrophic success. Join Date: Jan 2003 Location: Oklahoma Posts: 2,077	My experiences with bad boosters, are that most times you can hear a vacuum leak(hissing or sucking sound)or the motor will go into a rough idle when the pedal is pushed due to the tear in the diaphram in the booster.

09-22-2005, 11:36 AM	#8
farside847 Registered User Join Date: Feb 2005 Location: Bay Area, California Posts: 995	What is the diameter of the booster? If it is a small one from a car it will not perform as well as the larger truck ones... __________________ 1971 Chevy C10 Shortbed Stepside, 307v8 3-on-the-tree

09-22-2005, 10:24 PM	#9
modifiedGMC Registered User Join Date: Oct 2004 Location: Walker Co. Alabama Posts: 474	JHOW66 & 70BLOWNGMC: the pedal does sink some when the engine starts. I compared it with my wife's IROC and my pedal felt almost but not quite as hers felt before and after cranking the engine (my booster came off an 86 IROC) PUTTER: so what you are saying if my booster isn't working the oak tree will stop me or the water will soften the stop? SNEAKYSNAKE: The engine doesn'tidle rough and you can't hear any hissing. The booster is off an 86 IROC Z28 and the master cylinder is off a 79 cadillac. It's possible the master cyl. could be stiff but not likely. __________________ If it don't fit, force it,...If it still don't fit , modify it 'till it does fit !!!



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