This points to fluid and master cylinder problems. Unless you are extremely lucky and it's just fluid, this needs to get to your car repair shop. Even if it's just fluid, I highly recommend a brake fluid system flush which is more of a foul weather thing we do so often. That flush will also do the usual bleed to get the air out.
When I tap my brakes they sink to the floor and won't come back up until I tap them again. It's a 1995 f350. Thanks