i just had a booster problem that i fixed today... the one way vavle was on wrong... saved me 100+ bucks noticing that... the things people do..... but anyway... disconnect the vaccuum line from the booster, leave it connected to the intake manifold... then stick your finger over the other end with the one way vavle still hooked up.... the engine should almost stall (will if you dont let your finger off the tube), if you feel the vaccuum going through, you've solved your vaccuum question (dont put your lip up to the tube, and dont ask lol stupid testing ideas haha) but anyway, if you dont feel alot of vaccuum, disconnect the one way vavle, and check the intake manifold vaccuum tube again... if you feel it now, replace the one way vavle... if all of that still leaves you there... id replace the booster/master cylinder ... cuz if the master is old, it most likely will go soon after the new booster is put on, so why not knock it all in one shot... other things to follow with are bleed all of the brakes... look/listen for air, look for milky fluid or debris... and keep bleeding until you get back to clear fluid.... but all in all id start with the vaccuum line, then move to bleeding the lines out, and then worry about master/booster... start with the free or cheap stuff... cant hurt to do/replace anyway.