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RPMs drop when Brakes applied

dji1
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dji1 Enthusiast

dji1

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94 Silverado 350

 

When I apply the brakes, the rpms drop. In park or neutral the drop is approx 150 rpms and in reverse or drive approx 250 rpms. When I release the brake, the rpms go back up to normal.

 

I installed a new booster (rebuilt) last august 2025. Didn't have any problems until a couple of weeks ago. When I was working on the throttle body issues last month, I noticed some leaking around the booster vacuum line, so I replaced it. I have gone back and re-tightened the hose clamps and don't hear or see and leaks. 

 

I don't think the booster itself is leaking internally. The brake feels like normal and it stops ok. The pedal doesn't go to the floor, and I don't hear any hissing. When the truck is off, I still have pressure, so I guess the check valve is, ok?

 

I do question the rubber check valve grommet. Maybe its leaking around it? The replacement hose was just a section of new hose.  I plan to check it some more and test the check valve.

 

When I rebuilt the throttle body, I did replace the idle air control valve with another one I had. It was almost new, so I don't think that is a problem. 

 

Any other ideas or test suggestions.

 

Thanks

 

 

davester Senior Enthusiast

davester

Posted
Posted

maybe just plug the vacuum line to the booster, to rule out that path as being the cause of the rpm change?

dji1 Enthusiast

dji1

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I can do that.  When I pull the check valve off, I will plug the line going to the intake.

dji1 Enthusiast

dji1

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I did plug the vacuum line and the rpms where ok.  I also tighten the booster line going into the intake. It was a bit loose and that didn't make a difference.

 

 

asilverblazer Senior Enthusiast

asilverblazer

Posted
Posted
On 6/13/2026 at 4:30 PM, dji1 said:

I did plug the vacuum line and the rpms where ok.  I also tighten the booster line going into the intake. It was a bit loose and that didn't make a difference.

The booster probably failed internally. (Applying the brakes is basically creating a vacuum leak.)

dji1 Enthusiast

dji1

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I think you are right the brake booster must be leaking internally. I have tried everything else. Yesterday I replaced the check valve thinking it maybe wasn't closing all the way. Didn't really believe it since the booster held vacuum when the truck was off.  I even bought an original gm check valve from Rockauto. 

 

That booster isn't even a year old. O'Reilly's lifetime warranty. 

 

I am not looking forward to removing/installing that brake pedal clip again.

 

Good news is that my truck is running great after the throttle body re-built issues.

 

Thanks for the support.

 

asilverblazer Senior Enthusiast

asilverblazer

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Posted
9 hours ago, dji1 said:

I am not looking forward to removing/installing that brake pedal clip again.

Everything's always easier the second time!

dji1 Enthusiast

dji1

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It took me less than 5 minutes to remove the brake pedal clip. It is easier when you know how to do something.

 

Waiting on the replacement booster. Hopefully it will arrive sometime today. 

 

 

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