Revs drop when depressing brake

[Brit01]

Lately I have noticed the revs dropping when braking especially when at lights stopping suddenly.
Today I did some testing.
Checked the piping and changed the check valve from the vacuum source manifold.

When idling I depressed the brake repeatedly and each time the revs dropped until almost stalling.
Quick blip on the throttle and revs back up again.
If I start the engine with foot on the brake it drops and softens which I expected.


Could this be the drum failing? A small hole in the diaphragm?

Thanks