1980 Cobra - Idle speed increases when pumping brakes ??

[at_the_junkyard]

Weird...if I pump the brakes rapidly when in park the idle speed will increase by 200 or 300 rpm and then settle back down.

Suspect maybe a leaky vacuum booster or vacuum leak ? I pulled off the check valve and it appears to be sealing OK.

I recently did some work on the lines and the brakes do still feel a slightly "spongy". Thought maybe they just needed a bit more bleeding but maybe this has something to do with it ?

Anyone experienced this or have any thoughts ?

Thanks in advance.

[rob342]

I've had cars do that. Never figured it out, never got worse, never caused any problems. My mark 8 did it, my mustang does it slightly, and my 2002 pt cruiser did it a lot.

[wraithracing]

That generally an indication of a vacuum leak. Doesn't necessarily mean that the leak is the booster or it's line, but that there is a vacuum leak and when you push the brakes and the booster attempts to refill with vacuum the idle surges due to the vacuum loss. I would recommend checking all your lines and connections until you find the leak.

As for the brakes, I would recommend a full flush and bleeding of the brakes if they are mushy. That is an indication of air in the brake lines and needs to be bled out.

Good Luck!

Trey

[FM2NOTCH]

probably normal, my 85 does the same thing, a bad booster or leak will typically make the pedal hard to push,

[jessesandy]

My other car, a 90 Integra, does the same thing and I've decided to call it normal.
Besides, I got no good reason to pump the breaks real fast when I'm parked anyway.
Thanks, Todd H.

[Travis T]

My 84 GT has always done that.

[at_the_junkyard]

Thanks for all the replies !!

I could not find any vacuum leaks and from all the responses I am thinking this may be a normal dynamic as well.

However I did finally bleed some more small air bubbles out of the system and the brakes are firm and working great now...on to the next problem

Cheers,

[JACook]

It's perfectly normal, other than perhaps being an indication that your idle mixture is a bit fat. Every time you stroke
the brake pedal, the brake booster vents air into the assist side of the diaphragm, and when you release the pedal,
that air must then be sucked out again. So in that sense, the brake booster is an intentional vacuum leak.

[83gtstang]

I would pump the brakes on my car for fun, the idle did go up. Not sure why, but be sure your master cylinder is not leaking fluid into your booster. This will be sucked up into the booster and through the vacuum lines into the intake and burn causing your engine to smoke. You can sometimes see a wet dribble down thebooster where the master cylinder and booster meet. You will often need to bleed the brakes and add brake fluid when this happens.

[stevecap04]

My truck has this issue, but truck would die if i didnt double pump it. never figured main source replaced head, carb, deleted all emissions and now sits at 750 all day.