Brass musical instruments, such as trumpets, tubas, baritones and mellophones, work by depressing valves. The valves are pistons with holes in different positions on the piston to allow air to pass through. Pressing the valves in combination creates different tones. Brass instrument valves, though metal, are delicate and can be easily damaged. The slightest dent in a valve casing or dent in the actual piston can cause the valve to be stuck in place. Lack of use can result in the valves drying out, which could also cause them to become stuck in place. In either case, a couple of standard techniques can free a brass valve.
Unscrew the valve cap. Sometimes valves can become stuck when the valve caps are screwed on too tightly. If the cap will not unscrew, put a soft cloth around the cap and gently use pliers or a gripping device to unscrew the cap. This should be done with the utmost caution so as not to cause damage to the valve or the valve cap.
Oil the valves. It seems like an obvious thing to do, but the primary reason for a stuck valve on a brass instrument is because the valve is dry. A couple of drops of valve oil through the bottom of the valve casing, if the cap will not unscrew, or at the top of the valve, if the cap will unscrew, should be plenty. It is, of course, best to oil the valves outside the instrument. However, if the valve is stuck, oiling through the bottom or top of the valve casing is the only option. Rotate the instrument around a bit to allow the valve oil to trickle down the entire valve casing.
Check for dents in the valve casing. If oiling the valve did not free it, inspect the instrument for dents in the valve casing. Even the slightest dent can trap a valve. If the valve casing is dented, it is highly recommended that you take the instrument to a respectable brass instrument repairman, who have special tools, such as heating instruments that can expand the valve casing ever so slightly to free the valve without causing further damage to the valve casing.
Proper routine maintenance, such as regular oiling and cleaning and careful handling of a brass instrument will keep brass valves working properly. Clean the instrument at least once a week and always store it in a case.
If you have any doubt that you can repair the instrument, call a repairman. Any additional damage you cause to the valve will only add to your final repair bill.