Am having doubts about the hydraulic brakes on my '76 Impala.
When I first apply the brakes, at any speed, I would say, "fine", meaning it's working OK. ...But....

When I release pressure a bit, then continue to depress, the pedal goes down about a quarter inch or so, and keeps on going down as I apply more pressure, and I believe pedal will go ALL the way to the floor board.

Can anyone give me a detailed diagnosis ?
I thought the master cylinder was low but topping it off did not solve the loss of brake reserves.