It will not drain on the side where it collects without depressing the #2 piston and heroically blowing the water around to the other side of the circuit, depressing the valve again (without blowing) and allowing that water to - then - drain out the main slide...or (a real bother) pulling that hard-to-reach slide and dumping it "analog".
With many C tubas, F-sharp in the staff tends sharp, with no easy fix (unless someone doesn't mind substituting the wonky valve combination 5-2-3 for that pitch).
What I have found is that (though - undeniably - that pitch still tends sharp) it is EASIER to favor/lip it flatter WITHOUT water in the #2 circuit.
If your instrument is as described in the subject line - and your in-the-staff F-sharps seem to become sharper-and-sharper as you play - TRY (simply) letting the water out of that circuit. I suspect that - somehow - that water is (in effect) shortening that circuit.
Probably the same goes for E-natural with some same-configuration B-flat tubas...but there are not as many of those (Holton 345, M-W 195P, etc.)