It sounds like you have a leak somewhere in the system. A slow one but a leak just the same. Could be the MC, could be one of the wheel cylinders even. They should never slowly go to the floor and most times it's only a matter of time before they suddenly go and not slowling then you are in trouble.
Get all the wheels off and be sure the wheel cylinders aren't leaking, they have kits for those also, be sure the MC isn't leaking somewhere as sometimes they will leak on the inside of the firewall where you won't notice it.
If you still have the same problem follow the lines full lenght from back to front and see if there is a leak in the line somewhere.
They will never measure up to modern brakes but they aren't supposed to go to the floor either.
Deb