RPMs drop when Brakes applied

dji1 · June 8

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 · June 8

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

dji1 · June 8

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

dji1 · June 13

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 · June 15

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 · June 18

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.