Lots of stuff here.

Lets start with the easy first. You have to pump the pedal or it sinks 1" after you stop. If you don't have a leak its probably not the MC (could be - but probably not). Lets go step by step.

1) is there enough fluid in the MC res? - As pads wear the calipers are extended further and there is less fluid in the res - this can cause you to need to pump the brakes.
2) Wear - See 1
3) Tiny amount of air in the lines - air compresses - hydrolic fluid doesn't. When you step on the brake and there is air it will get squeezed in and the pedal will sink. This could be anywere - perform a good bleed and if it was air - this will fix it (assuming there isn't a leak).

So - go ahead and do the flush - bleeding from the farthest caliper and once clear new fluid is coming out of the bleeder move to the next farthest etc.

If that doesn't fix it - then start looking at the MC - these are fairly inexpensive - but you dont want to replace if you dont have to. Usually when the MC fails though - there will be a leak because its the fluid pushing past the rubber seals that causes you to lose brake pressure.

OK - the vacuum - You are on the right track and its good you know the VP works. Go ahead and repalce the lines from the check valve to the booster.

I'll need a pic of the cactus to know exactly what you are referring to. But if you have a firm pedal - you need more vac to the booster.

Does the pedal get better at higer speeds? not sure why that would be -