It would help to know what model it is.
My Cerveny 781 (virtually identical to a Miraphone 186) would now and then get some significant condensation in the bottom bow of the third valve circuit. In those cases I had this little ritual I went through of turning the tuba partly upside down, rotating it maybe 90 degrees in that attitude, and turning it upright. That would get the water into the main circuit and out through the (only) water valve. I can't possibly describe the ritual in detail.
Alternatively, do the usual "tuba spin": Being careful not to drop the tuba and ruin it (HIGH RISK HERE!), you spin it over and over in the correct direction to get the water to come out the bell. The "spin" should be slow and careful. Your music director should be able to show you this. Otherwise, ask a French horn player.
If it's in the 4th valve circuit, however, I'd guess you should be able to empty it by pulling the fourth valve slide -- and maybe inverting the horn. Again, BE CAREFUL. Don't let the horn slip out of your grasp.
Or ... Email Tom McGrady (sales@mackbrass.com) and ask him what his recommendation is. He will be happy to help, and in my experience he responds very quickly to any inquiries.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)