RPMs drop when Brakes applied

[dji1]

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]

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

[dji1]

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