Why does my RPM increase when I step on brake pedal?

[78K204wd]

Hi, I got a 78K20 with a 350 and Edelbrock 1406. My vacuum booster is connected to back of the carb with what appears factory fitting, and the vacuum advance is connected to drivers side port on carb lower front.

When I step on the brakes (while in park) the rpm increases.  Is my V booster leaking, and is that bad for engine performance?  My idle has been varying when driving around town.thanks bob

[bd]

You probably have a leaking brake booster diaphragm.  It's not good for performance - worse for stopping!  After you shut the engine off, step on the brake pedal.  Is there power assist for 2-3 strokes?

[velojym]

 

Wrong pedal!!!


 

[Irish_Alley]

Wrong pedal!!!


 

[78K204wd]

The brakes work great on this truck, that's one thing I didn't have to mess with.

[bd]

Generally, when idle quality is affected while stepping on the brake pedal, there is an unwanted vacuum leak through the booster.  Does idle quality improve if you disconnect and plug the booster vacuum hose from the manifold?

[78K204wd]

I am gonna try that next to see if that helps.   It seems to bog when I step on it after a bunch of tuning, and as I mentioned earlier - the rpm increases when I press the brakes (in park).  Perhaps the two are related, messing up my vacuum advance.

[LeftysRodandCustom]

Yes they are related. If there is a vacuum leak in the booster then it will cause a loss of vacuum when stepping on the pedal which in turn allows the distributor to advance as though you were at a partial open throttle position.

[Irish_Alley]

you should hear the booster hissing when you press on the brakes. if it does hiss than yes you have a leak

[wkeithwilson@gmail.com]

I'm not having the issue with high RPM when the brakes are pressed, but I do hear a "hiss" when I press them.  It literally sounds like the pedal is hissing.  Any recommendations (or links) on how to troubleshoot that?

[bake74]

I'm not having the issue with high RPM when the brakes are pressed, but I do hear a "hiss" when I press them.  It literally sounds like the pedal is hissing.  Any recommendations (or links) on how to troubleshoot that?

     Go to this link and look up your year of truck in the tech section, and read through the manual.  It describes how to troubleshoot brake boosters.