DodgeForum.com - 1998 Caravan Brake Problem

Greetings all. I need some ideas here to try to figure out what my van is doing and has been doing for a while. Right now my brakes are almost non-existent. The pedal will go right to the floor before any braking happens, and if I push too hard the wheels lock up. It just snowed a bit here so that makes things worse. The van has done this a few times before when it was still my dad's vehicle. Each time it did it the brakes were bled, and I believe at some point the master cylinder was replaced. Unsure if any other components were replaced. Each time everything seemed fine, until one day the pedal randomly goes right to the floor again and it all starts over. I don't see any leaks anywhere, and with my driveway covered in a light coating of snow I sat there pumping the brakes like mad to see if there was a leak somewhere, as the snow would make it obvious, but nothing. Pumping the pedal gets me a bit more pressure, but as soon as I let off completely, it'll go to the floor again. This is extremely frustrating and I certainly don't feel safe driving this thing in this condition for fear of not stopping when I really need to. Looking for any suggestions on what to have checked when I take it in to a repair place tomorrow.