The source of value of a currency is what the govt makes us do to get it. Not this thing about quantity of bonds. Bonds are issued only as a reserve drain. They allow non-interest bearing excess reserves to be transferred to another 'account' at the fed called 'tsy secs' that are interest bearing, thus supporting the fed's rate target. Float is an operating factor that requires defensive fed action.
Reserves are balances held at the fed.
Excess reserves are balances in excess of that required by the fed. The overnight rate falls. I don't call that a crisis. It could be a policy decision. Banks can always pay interest.