that doesn't sound normal!

shouldnt be like a duck call!

should be free flowing in one direction, and not flowing at all in the other

maybe it is blocked/jammed

when you jiggle in your hand you should be able to hear and feel the slide mechanism going back and forward inside it, it should move very easily, and if it is operating correctly it will not restrict flow much at all, should be like blowing in a maccas straw

essentially there is just a solid cylinder in that valve that slides from one end to the other depending which way air is flowing

you should not be blowing you should be sucking, the engine sucks through that valve to pull the diaphram in the brake booster, you should be simulating that by sucking on the engine side of the valve (by doing this you are pulling the cylinder inside the valve to the engine side, this is allowing flow through the valve, so actually this is the same as blowing from the brake side..... you should be able to suck through the valve from the engine side or blow through the brake side.... and not the opposite scenarios)

edit[] the whole engine rev when brake thing is not particularily dangerous, although its not great, essential you are accelerating and braking together, which obviously is going to increase your stopping distance. you should definately fix this problem as a priority, although personally i would still drive it carefully if i had no alterative.... maybe to a brake place so they could fix it!