Idle surge when applying power brakes - The 1947 - Present Chevrolet & GMC Truck Message Board Network

fatbass · 02-20-2019, 02:56 PM

I was noticing that when I push on the brake pedal when idling in neutral, the RPM increases about 2-300 rpm. This was during my weekly "start it and run through the gears until it reaches operating temperature".
I haven't yet cleaned off the snow to have a look underhood but I was wondering if this is common or if someone knows what the likely fix is. I'm guessing vacuum leak. Truck is a '71 C20 with power brakes.

Wrenchbender Ret · 02-20-2019, 03:45 PM

If it does it once it is normal. If it continues while you have your foot on the brake in means a ruptured diaphragm in the power brake unit.

**D.PASSMORE** · 02-20-2019, 03:55 PM

Power brake diaphragm pushes air back into intake runners, which leans Air/Fuel ratio, which caused slight RPM increase.

fatbass · 02-20-2019, 04:10 PM

Thanks to both of you. I'll pay close attention to see if it's momentary or continuous. It'd be bad if the booster is fubar.
If I fly through a stop sign, I'll post up crash pics.

02-20-2019, 02:56 PM	#1
fatbass Registered User Join Date: May 2012 Location: South Ogden, Utah Posts: 172	Idle surge when applying power brakes I was noticing that when I push on the brake pedal when idling in neutral, the RPM increases about 2-300 rpm. This was during my weekly "start it and run through the gears until it reaches operating temperature". I haven't yet cleaned off the snow to have a look underhood but I was wondering if this is common or if someone knows what the likely fix is. I'm guessing vacuum leak. Truck is a '71 C20 with power brakes. __________________ '71 C/20 350/T350.

02-20-2019, 03:45 PM	#2
Wrenchbender Ret Registered User Join Date: Oct 2005 Location: Overland Park, Ks. Posts: 5,229	Re: Idle surge when applying power brakes If it does it once it is normal. If it continues while you have your foot on the brake in means a ruptured diaphragm in the power brake unit.

02-20-2019, 03:55 PM	#3
D.PASSMORE One of the First Join Date: Jan 2000 Location: St. Augustine,FL. Posts: 4,059	Re: Idle surge when applying power brakes Power brake diaphragm pushes air back into intake runners, which leans Air/Fuel ratio, which caused slight RPM increase. __________________ DAVID PASSMORE ST AUGUSTINE ,FL ZIP 32084 '71 SWB K/10 CUSTOM DELUXE (Chili) '72 BLAZER K/5 Muscle Truck (Mule) '72 LWB K/20 CHEYENNE (Blackie)

02-20-2019, 04:10 PM	#4
fatbass Registered User Join Date: May 2012 Location: South Ogden, Utah Posts: 172	Re: Idle surge when applying power brakes Thanks to both of you. I'll pay close attention to see if it's momentary or continuous. It'd be bad if the booster is fubar. If I fly through a stop sign, I'll post up crash pics. __________________ '71 C/20 350/T350.

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