So be it, Tony…
However; the engine uses a prodigious amount of air even in idle - far beyond a leak loss. So braking effect should not vary with engine rpms as a leak indicator - manifold vacuum is a better indicator of expected braking effect.
At speed, when you lift foot to apply brakes; manifold vacuum increases to some 23’ Hg as throttle closes - increases braking effect. Idling in gear tends to lower vacuum - but coasting at low speed sort of keep vacuum at idle level.
A second consideration; the one-way valve is there to keep the highest vacuum reached inside the booster - enhancing braking at all times. With a failed one-way valve; braking effect varies directly with manifold vacuum…
However, checking for leaks is certainly pertinent - whether or not this influences braking. However; if the leak is inside the booster, it will not show up with an external leak test. A leaking booster will influence braking effects, though not necessarily rpms dependent.
A leaking booster is detectable by a constant hissing at the brake pedal rod when the pedal is held down…
sxj6 85 Sov Europe (UK/NZ)