Hi,

The instructions you are using for testing the booster are correct, up to point.

You need to two more things or one depending on the order you like.
Shortly after stopping the engine with everything hooked up you should be able to pull on the check valve and it’s grommet to try to get it out. It should hold a vacuum in there for quite a bit. It will hiss air in if it’s holding like it should.
I cannot say that there is a specification for it but, if the chamber is not holding some vacuum, then this much vacuum has to be made up before the engine will run smoother or start up faster.

The next one is to isolate the brake booster from the car intake manifold entirely to see if the roughness of engine idle does goes away.
This would help confirm that either there is a leak in the booster diaphragm or in the hose and if the check valve is holding vacuum.
If that makes the engine run better then you are on to something on that side for sure.

I wanted to tell you this because getting the booster exchanged out is quite a bit of under the dash work. You don’t want to do this unless you are sure that’s leaking.
It can be rather difficult to get to all four nuts without extensions and swivel joints.

My back is not inside a contortionist body by any means!

Phil