The photo doesn't clarify the problem; a side view would have been appropriate.
First, check that each piston is in the correct cylinder and orientation. To do this, blow air through the horn while depressing the pistons in these combinations — none, 1, 2, 3, 1+2, 1+3, 2+3, all. Any back-pressure indicates a problem.
Most pistons are numbered, with the number facing the player in correct position. When they aren't numbered, it's easy to determine which is the 2nd as the 2nd valve's tubing loop is at a right angle, and then the others can be found by trial.
If the water key is really the problem, usually the stopper cork (sometimes made of soft rubber) is damaged or missing, but is easily replaced.