Suddenly my brake pedal is very soft - goes almost to floor before the brakes engage.

Now if I have the engine off, pump the brakes once, then the pedal is normal.

So its only soft when engine is running.

Is this the master cylinder problem I've read about on the forums?

Fluid level is fine.

Thanks,
2000 Silhouette Owner

Give your brakes a good inspection especially the rears. If there out of adjustment the pedal will be low.

If everything looks normal, no leaks, and properly adjusted the master might be your problem.