car brakes well with light brake pressure. If car is in park with a little more brake pressure the brake pedal will slowly go down to floor but will repump back to normal position. There are no leaks on calipers, lines or ground. The level of brake fluid in master cylinder does not go down. 58,000 miles
Two possibilities come to mind. First, a leak in the master cylinder. Second, a bad vacuum booster. Don’t live with it, have it checked out and taken care of.
If the fluid is not going down, I would vote for a bad master cylinder. The fluid is leaking backward past the piston, and goes back into the fluid reservoir.