95 taurus with bad brakes

Ive got a 95 taurus gl with 115K miles on it, and my brakes are really weak. They are so bad that pushing them all the way to the floor instantly while going about 30 on very wet roads cant even create wheel lock or cause abs to come on. ABS works cause i hit a patch of gravel while braking fully and it came on in the one wheel that locked. It is not hard to push down brakes like there is a vaccuum leak and it never becomes hard to push down even if i sit there and pump the brakes while in neutral with engine running. Brakes are not at all spongy, however when car has sat overnight, brakes are very weak and cannot even stop car from just rolling sometimes (yes it has increased idle due to engine being cold and just started) but the brakes are not hard to push when it does this either. I plan on replacing the master brake cylinder on friday but is this the part that is bad and if so, do i need to also replace the brake booster or any other parts? Finally will a cheap remanufactured master cylinder work fine or what should i buy? Thanks for any help with this problem.

I went around and around with the brakes on my 1993 doing similar stuff...everything finally was solved when I had the brake booster, a valve and the master cylinder replaced. Be careful about air bubbles in the brakes. These can cause big problems. Take it to a shop that's got experience servicing newer cars with these more complicated brake systems.
As for the master cylinder, cheap ones aren't usually a good idea since these cars are so touchy. NAPA is a good buy. Also, use synthetic brake fluid, it's really seemed to work well for me...I used the Valvoline Syn Power stuff in the gold bottle.