RPMs go down when I press the brake - The 1947 - Present Chevrolet & GMC Truck Message Board Network

skokie · 11-22-2004, 12:51 AM

I just noticed this tonight. I pressed on the brake and the rpms went from 750 to 650. I let off and its back to normal. Setup is a sbc with power brakes. Any suggestions?

68w/sbc406 · 11-22-2004, 12:53 AM

thats normal. i think it has something to do with the booster using the vaccum off of the motor.

skokie · 11-22-2004, 12:53 AM

Thanks much.

Arctic Gearhead · 11-22-2004, 01:32 AM

In regards to your idle change when applying the brake - a modest change is somewhat normal but 100 rpm may be a little excessive. You can ascertain whether or not it is the booster by clamping the vacuum line to the booster with a pair of hose pliers and then applying the brake again. No change with the line clamped and 100 rpm change with the line not clamped would definately confirm that the brake booster is the source of the change. Many times when the diaphram in the booster is leaking, if you apply the brake and put your ear near the back of the booster you will hear it leaking. Also with a vacuum gauge it would be normal when applying the brake to note a momentary drop in vacuum upon application, but the vacuum reading should quickly stabilize at or near where it was.

It could also be that, because of camshaft profile or other issues, you do not have enough vacuum available. This can be solved relatively simple by using a vacuum reservoir sold by Summit, Jegs, or somewhere similar. Speaking of those vendors they also sell a book called "The Welding Handbook" (or something similar to that) - it is a really good book.

Good luck with your truck!

11-22-2004, 12:51 AM	#1
skokie Registered User Join Date: Oct 2003 Location: San Jose Ca. Posts: 3,354	RPMs go down when I press the brake I just noticed this tonight. I pressed on the brake and the rpms went from 750 to 650. I let off and its back to normal. Setup is a sbc with power brakes. Any suggestions? __________________ RIP to my son Dylan. 5/17/2002 - 8/18/2018 If you want to know more about rare forms of pediatric cancer or if you want to help fund pediatric cancer research, please check out the website for the non-profit I started. https://slothforpedcancer.org 68 build thread http://67-72chevytrucks.com/vboard/s...d.php?t=223234

11-22-2004, 12:53 AM	#2
68w/sbc406 Got Light Emitting Diode? Join Date: Apr 2004 Location: Newberg, OR Posts: 2,485	thats normal. i think it has something to do with the booster using the vaccum off of the motor. __________________ 1968 1/2 ton 2wd lwb 6 lug disc and bags up front. Next c notch and rear bags '02 Chevy Silverado LT ext cab short bed 2WD 5/7 drop on 22's(the family car) 1993 Cherokee work ride/weekend wheeler

11-22-2004, 12:53 AM	#3
skokie Registered User Join Date: Oct 2003 Location: San Jose Ca. Posts: 3,354	Thanks much. __________________ RIP to my son Dylan. 5/17/2002 - 8/18/2018 If you want to know more about rare forms of pediatric cancer or if you want to help fund pediatric cancer research, please check out the website for the non-profit I started. https://slothforpedcancer.org 68 build thread http://67-72chevytrucks.com/vboard/s...d.php?t=223234

11-22-2004, 01:32 AM	#4
Arctic Gearhead Registered User Join Date: Nov 2004 Posts: 3	Diagnosing Change in Idle from Brake Application In regards to your idle change when applying the brake - a modest change is somewhat normal but 100 rpm may be a little excessive. You can ascertain whether or not it is the booster by clamping the vacuum line to the booster with a pair of hose pliers and then applying the brake again. No change with the line clamped and 100 rpm change with the line not clamped would definately confirm that the brake booster is the source of the change. Many times when the diaphram in the booster is leaking, if you apply the brake and put your ear near the back of the booster you will hear it leaking. Also with a vacuum gauge it would be normal when applying the brake to note a momentary drop in vacuum upon application, but the vacuum reading should quickly stabilize at or near where it was. It could also be that, because of camshaft profile or other issues, you do not have enough vacuum available. This can be solved relatively simple by using a vacuum reservoir sold by Summit, Jegs, or somewhere similar. Speaking of those vendors they also sell a book called "The Welding Handbook" (or something similar to that) - it is a really good book. Good luck with your truck!



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