Effects of brake booster on idle on 230SL

[perry113]

Hello my forum members.
I am help my friend with his 1963 230SL roadster.
This car has suffered erratic an idle for a long time.
Linkage adjustments are good.
I disconnected the vacuum line to the brake booster and capped off the intake manifold and found the idle to smooth out and run very smoothly. I recommended a booster replacement.
He had another booster rebuilt. I just swapped in the rebuilt booster and found the car to run very similarly to the old booster. Idle hunting and fluctuation. I then capped off the intake manifold booster provision and the idle is good. My next move will be is to swap out the 2 piece vacuum line off of another 230SL which happens to also have a brake booster issue. The second car idles smoothly but the idle increases when the brakes are applied.
I have heard word its a waist of time having these boosters rebuilt. The success rate of a good booster is low.
My question is what affects on idle are there with a "perfect" booster? I assume if your booster is 100% with no leaks then the idle set up should not differ whether you booster line is active versus capping of the provision on the intake manifold. We checked for vacuum leaks and can't seem to locate any.
Curious as to your thoughts.
Cheers !
Peter

[Vander]

If the idle changes while pressing the brake pedal you have a booster issue. I've had a rebuilt unit I had to send back a second time.

You can use a handheld vacuum pump with gauge hooked up to the booster, put vacuum on the booster if it holds vacuum booster is good. If it loses vacuum booster has an issue.

[stickandrudderman]

There are re-built boosters and there are rebuilt boosters.......