Topic: Idle increases as brakes applied

[V8_bloke_28]

Hi people
Dads 67 mustang has developed an issue where the idle increases when the brakes are applied.
We thought it was the booster but that?s been refurbished and checked twice.
Engine is a stock 289 and has 20hg vacuum at idle.
All hoses look good and engine runs perfect without brakes on.
Any suggestions would be appreciated.
Cheers

[1966KGT]

Have you isolated the booster by unhooking and plugging it? Just a place to start, my experience has been with vacuum leaks is high idle. Could be a leak in another area and when brakes applied puts a demand on vacuum of engine and the weak sealing area will leak? just my thoughts to start. Anyone else?

[J_Speegle]

Yes sounds like a vacuum leak.

Plug off the vacuum hose from the intake will likely result in no change when the brake is pressed.

Pulling a vacuum using the check valve connection at the booster will likely be impossible or not hold a vacuum once established

[hopey]

You say booster is good. Did you verify the check valve is holding?

[V8_bloke_28]

Thanks for your ideas.
Check valve  and grommet is brand new.
Dad called a mechanic over to see what he thinks and he believes it?s the booster.
Looks like it?s coming out for a 3rd time.
This time it?s gonna get thrown at someone!
Cheers

[Drew Pojedinec]

I hear this a lot.


Me: is xyz good?
Customer: Its brand new.
Me: did you test it?
Customer: no.


Assume everything is bad and proceed accordingly.

[Bossbill]

Thanks for your ideas.
Check valve  and grommet is brand new.
Dad called a mechanic over to see what he thinks and he believes it?s the booster.
Looks like it?s coming out for a 3rd time.
This time it?s gonna get thrown at someone!
Cheers

An SAE mechanic or the guy down the street who owns a hot rod (built by someone else)?

Replace the grommet and valve first. A lot cheaper and easier than pulling the booster.

The boosters usually leak to the inside. With the engine running push on the brake and use a section of  hose to your ear and near the push rod to see if you hear a hiss at the firewall.

[jwc66k]

Assume everything is bad and proceed accordingly.

Then you'll never get anything done.
Jim

[Countrysquire]

I hear this a lot.


Me: is xyz good?
Customer: Its brand new.
Me: did you test it?
Customer: no.


Assume everything is bad and proceed accordingly.

Yep. Lost an hour of my life deciphering a electrical schematic written in German trying to figure out why the headlight bulb I just replaced on a 911 still wasn?t working. Turns out the new bulb was bad right out of the package. Poor troubleshooting on my part by assuming the new bulb was good.

[Bossbill]

My wife's 911 has a license plate that starts with AUF. It just sounds German. That's also the sound I make when I have bend down that far to get in it...