Power Brake Booster Question - The 1947 - Present Chevrolet & GMC Truck Message Board Network

SLS · 07-04-2005, 01:26 AM

I971 Chevy C-10

Is there a way to 'test' a Power Brake Booster to see if it is functioning properly?

My Master Cylinder is fine (less than 200 miles on a new unit), and I have no leaks in the brake lines...and all wheels have been bled to the max...no air.

But my brakes, although they do stop the truck well, feel like like they are unassisted. The pedal does not do the old 'pump up' on startup only to fall flat after braking...and the pedal does not feel 'hard' either. I just feels like hydraulic brakes with no booster.

The vacuum line that comes in from the base of the carb is pulling vacuum (but I have not measured it).

Is there a quick way to check the booster (in place on the truck) to determine if it is not functioning correctly?

Thanks for any info! And Happy 4th of July everyone!

krue · 07-04-2005, 01:49 AM

Haynes manual has a check for it. I don't remember off the top of my head, but I'll look in the morning if I can remember to.

Zeke · 07-04-2005, 08:41 AM

It should hold vacuum after the engine is shut off. Start it, shut it off (don't touch brake) then with the motor off step on the brake pedal it should make a hissing sound when you step on the brake, you should feel the assist which will get less each time you step on it. Make sure the check valve is good first it holds the vacuum in the booster after motor is shut off.

jermasaurus · 07-04-2005, 01:45 PM

First, get all the vacuum out of it by pressing the pedal a few times. I think with most you can apply the brakes with the motor off, start the motor, and you should be able to feel the pedal move down further once the vacuum is assisting it.
Maybe somebody can tell otherwise, but I thought you could put a larger diameter booster on there for more assist? Maybe one from a 3/4 or 1 ton?
-Jeremy

SLS · 07-05-2005, 11:47 AM

Thanks for the replies, guys.

I tried the things that were suggested, but it just doesn't feel like it is supposed to. The only thing I can do that makes it feel like there is any difference at all between when the engine is running , and when it is shut off,... is when I put pressure on the pedal with the engine off it feels firmer until I start the engine...and then the pedal drops just a little bit (maybe a 1/2 inch) and then is feels 'softer'....but not able to 'pump it up'.

The check valve is working good too...only allows air to flow in one direction....and holds vacuum too (no 'leak-off').

Zeke · 07-07-2005, 04:54 AM

Not being able to check myself but I don't think power brakes ever feel real hard except when you bleed all the vacuum off. If they stop the truck ok and don't drop to the floor when you hold them they should be fine. If they do keep going to the floor then it is something else in the system. The booster just assists in braking and the rest of the system works just like none power brakes. Not being there makes it kinda hard to tell if there is a problem.

smokekiki · 07-07-2005, 08:11 AM

Start the truck,build the vacuume up,shut it off,wait 5 min,and pull the hose off.If it is working,you will hear the rush of air.

SLS · 08-05-2005, 07:18 PM

Sorry for the delay in getting back to you guys....my baby daughter arrived June 6th....and today was the first day I was able to mess with the old bucket again!

I did what smokekiki suggested,....and heard the 'woosh' of air being released....so I guess it is working correctly. Maybe I am expecting more from a 34 year old brake system than I should. It does seem like my old 73 Nova used to have much more powerful brakes though....but maybe my memory ain't so hot anymore.

Thanks for the help and suggestions!

Goose · 08-06-2005, 01:31 AM

Are you taking vacuum directly from the intake manifold? Not sure what vacuum line you are referring to " at the base of the carb." Most intake manifolds have a plug behind the carb as a potential source of vacuum. At least that is the one I used for my power brake conversion and I don't seem to have that "soft pedal" syndrome your experiencing.

Senor_paco · 08-07-2005, 11:32 AM

my brakes felt a bit firmer after replacing the original hoses with braided stainless. At $75 for all three, it made a big difference too. Thought I'd offer that up too.

07-04-2005, 01:26 AM	#1
SLS Finish the'71..or bust! Join Date: Jun 2003 Location: Nashville, TN. Posts: 441	Power Brake Booster Question I971 Chevy C-10 Is there a way to 'test' a Power Brake Booster to see if it is functioning properly? My Master Cylinder is fine (less than 200 miles on a new unit), and I have no leaks in the brake lines...and all wheels have been bled to the max...no air. But my brakes, although they do stop the truck well, feel like like they are unassisted. The pedal does not do the old 'pump up' on startup only to fall flat after braking...and the pedal does not feel 'hard' either. I just feels like hydraulic brakes with no booster. The vacuum line that comes in from the base of the carb is pulling vacuum (but I have not measured it). Is there a quick way to check the booster (in place on the truck) to determine if it is not functioning correctly? Thanks for any info! And Happy 4th of July everyone!

07-04-2005, 01:49 AM	#2
krue Designated A-hole! Join Date: Mar 2003 Location: South Mississippi Posts: 36,448	Haynes manual has a check for it. I don't remember off the top of my head, but I'll look in the morning if I can remember to. __________________ "If it ain't broke, fix it 'til it is!" Being stupid ain't illegal. We're Still Debt Free Except for the House!!! www.daveramsey.com 70 GMC SWB Stepside project "Green Booger" soon to be 6.0l/4l80e 93 S-10 "Poppaw's Truck"

07-04-2005, 08:41 AM	#3
Zeke Active Member Join Date: Jun 2002 Location: L.A. Lower Alabama Posts: 453	It should hold vacuum after the engine is shut off. Start it, shut it off (don't touch brake) then with the motor off step on the brake pedal it should make a hissing sound when you step on the brake, you should feel the assist which will get less each time you step on it. Make sure the check valve is good first it holds the vacuum in the booster after motor is shut off. __________________ 1969 C10 350 3spd 1995 GMC 3500 XC 350 dually When Thomas Edison worked late into the night on the electric light, he had to do it by gas lamp or candle. I'm sure it made the work seem that much more urgent. George Carlin

07-05-2005, 11:47 AM	#5
SLS Finish the'71..or bust! Join Date: Jun 2003 Location: Nashville, TN. Posts: 441	Thanks for the replies, guys. I tried the things that were suggested, but it just doesn't feel like it is supposed to. The only thing I can do that makes it feel like there is any difference at all between when the engine is running , and when it is shut off,... is when I put pressure on the pedal with the engine off it feels firmer until I start the engine...and then the pedal drops just a little bit (maybe a 1/2 inch) and then is feels 'softer'....but not able to 'pump it up'. The check valve is working good too...only allows air to flow in one direction....and holds vacuum too (no 'leak-off'). Last edited by SLS; 07-05-2005 at 11:48 AM.

07-07-2005, 04:54 AM	#6
Zeke Active Member Join Date: Jun 2002 Location: L.A. Lower Alabama Posts: 453	Not being able to check myself but I don't think power brakes ever feel real hard except when you bleed all the vacuum off. If they stop the truck ok and don't drop to the floor when you hold them they should be fine. If they do keep going to the floor then it is something else in the system. The booster just assists in braking and the rest of the system works just like none power brakes. Not being there makes it kinda hard to tell if there is a problem. __________________ 1969 C10 350 3spd 1995 GMC 3500 XC 350 dually When Thomas Edison worked late into the night on the electric light, he had to do it by gas lamp or candle. I'm sure it made the work seem that much more urgent. George Carlin

07-04-2005, 01:45 PM	#4
jermasaurus Registered User Join Date: Dec 2004 Location: New Lenox, IL Posts: 442	First, get all the vacuum out of it by pressing the pedal a few times. I think with most you can apply the brakes with the motor off, start the motor, and you should be able to feel the pedal move down further once the vacuum is assisting it. Maybe somebody can tell otherwise, but I thought you could put a larger diameter booster on there for more assist? Maybe one from a 3/4 or 1 ton? -Jeremy

07-07-2005, 08:11 AM	#7
smokekiki Mike Join Date: Jan 2003 Location: west chester pa Posts: 2,473	Start the truck,build the vacuume up,shut it off,wait 5 min,and pull the hose off.If it is working,you will hear the rush of air. __________________ 70'c/10, 71 suburban4x4 402bb, 72suburban 4/6 drop, 72k/5 4x4 blazer 4" lift 35 tires

08-05-2005, 07:18 PM	#8
SLS Finish the'71..or bust! Join Date: Jun 2003 Location: Nashville, TN. Posts: 441	Sorry for the delay in getting back to you guys....my baby daughter arrived June 6th....and today was the first day I was able to mess with the old bucket again! I did what smokekiki suggested,....and heard the 'woosh' of air being released....so I guess it is working correctly. Maybe I am expecting more from a 34 year old brake system than I should. It does seem like my old 73 Nova used to have much more powerful brakes though....but maybe my memory ain't so hot anymore. Thanks for the help and suggestions! Last edited by SLS; 08-05-2005 at 07:39 PM.

08-06-2005, 01:31 AM	#9
Goose Goose Join Date: Jun 2000 Location: Austin Posts: 142	Are you taking vacuum directly from the intake manifold? Not sure what vacuum line you are referring to " at the base of the carb." Most intake manifolds have a plug behind the carb as a potential source of vacuum. At least that is the one I used for my power brake conversion and I don't seem to have that "soft pedal" syndrome your experiencing. __________________ '72 GMC short-step 350/350

08-07-2005, 11:32 AM	#10
Senor_paco Registered User Join Date: Mar 2003 Location: San Antonio, TX Posts: 441	my brakes felt a bit firmer after replacing the original hoses with braided stainless. At $75 for all three, it made a big difference too. Thought I'd offer that up too.



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