Vaccum issue

[shonto777]

so here's is probably a goofy question.i have a 1983 K10 with 383 stroker long bed with dual tanks, with less than. 100k miles. Why would I hear a significant change in My idle with my foot on the brake? I go from sitting there in neutral listening to my truck at idle, to pushing the brake pedal, and hearing the Idle pick up. Not crazy like redlining or anything but def noticeable to my ears. Is that a vacuum issue, because the master cylinder works off vacuum right?.

 

[fast 99]

Applying brakes does allow vacuum to flow into the power brake unit. That's how the system assists with braking. Engine speed should only momentarily increase when the brakes are first applied. If the higher engine speed continues during brake application there is likely an internal leak. Most of the time a leak can be diagnosed by letting the vehicle sit [engine off] for a few minutes to an hour or more. Pull the vacuum check valve at the booster. Should hear a hiss as air rushes into the booster. If not, booster and or check valve is bad.

 

[gmbellew]

Applying brakes does allow vacuum to flow into the power brake unit. That's how the system assists with braking. Engine speed should only momentarily increase when the brakes are first applied. If the higher engine speed continues during brake application there is likely an internal leak. Most of the time a leak can be diagnosed by letting the vehicle sit [engine off] for a few minutes to an hour or more. Pull the vacuum check valve at the booster. Should hear a hiss as air rushes into the booster. If not, booster and or check valve is bad.

Doesn't the cruise control also have a vacuum switch to kill the cruise when you hit the brakes?

 

[fast 99]

Don't know if that switch is vacuum or electric but yes, cruise is killed by brake.

If you suspect a problem, temporarily plug the vacuum hose to diaphragm.