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02-20-2019, 02:56 PM
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#1 |
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Registered User
Join Date: May 2012
Location: South Ogden, Utah
Posts: 172
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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.
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'71 C/20 350/T350. |
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02-20-2019, 03:45 PM
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#2 |
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Registered User
Join Date: Oct 2005
Location: Overland Park, Ks.
Posts: 5,228
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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.
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02-20-2019, 03:55 PM
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#3 |
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One of the First
 
Join Date: Jan 2000
Location: St. Augustine,FL.
Posts: 4,054
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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.
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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) |
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02-20-2019, 04:10 PM
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#4 |
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Registered User
Join Date: May 2012
Location: South Ogden, Utah
Posts: 172
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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.
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'71 C/20 350/T350. |
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