Brake Vac Line: Why does it have a one-way valve?
 

Why does the vacuum line for the brakes on my SC have a one-way valve?
Am I to assume that the valve opens only when the engine is pulling a vacuum?
What are the symptoms of a bad one?

Its actually there to keep the loss of vacuum to your booster on low/no vacuum situations. Think of it more like keeping a steady amount, not fluctuating like the vacuum in the engine does under different throttle conditions. You shouldnt notice any "engine" issues, like bad idle, power, etc if it is bad, unless your booster is leaking. Simply pinch off the lines on either side with the engine running, you should notice no change in RPMs.