The valves should be trimmed to just barely cover the slot they're on top of. Take care when reassembling the harmonica that the comb does not interfere with the free operation of the valves. If the comb keeps the valve from lifting during play, the reed won't sound, or won't sound right. Valved bends are a little different than normal diatonic bends. During a normal bend, both reeds in the chamber can participate to produce the characteristic gutsy sound.
Valved bends are more delicate and require more control to execute cleanly and clearly on pitch.
Only one reed participates in the generation of the sound, since the other reed is blocked by the valve. It is especially important not to attack the bend hard when you initiate it, otherwise it will choke off and not sound. A pinching of the lips will not produce a good valved bend. Valved bends can be done on the chromatic, as well as a valved diatonic. But, with a little practice valving your own harp will only take 5 or 10 minutes.