Before putting pistons back in casing, be sure there is no lint left in the ports or later this will come out and foul the action. Also, before putting the pistons back in casings, place a few drops of oil on the pistons. Use a light, high grade valve oil. Many players get good results by putting a few drops of oil on the piston, then running cold water over it and working it into the casing. Some players prefer to use water or saliva on new, tight-fitting valves, but generally speaking a fine oil is best. On valves which are worn, a heavier oil can be used, as this heavier oil helps seal up the excess clearance which has worn between the walls of the piston and casing.
Piston valves are very accurately fitted with clearance from .0012″ to .0018″ (one and two-tenths of a thousand to one and eight-tenths of a thousandth of an inch). The clearance on a side is just half this, or about .0008″ (8 ten-thousandths of an inch) or the diameter of an average human hair. This does not leave much space for the film of oil, but a fine grade of oil will work all right. Valves that wear until the clearance reaches .0025″ to .003″ are liable to leak. Only a heavy oil
which helps close up the excess clearance will make them playable.
An instrument not properly cleaned and oiled will corrode after months of disuse. Valve caps will stick and cannot be unscrewed. Don’t use strong-arm methods to loosen them. Don’t use pliers or pipe wrench, for you’ll mar and break the instrument. Sometimes tapping with a wood mallet or wood hammer will be enough to break the corrosion loose sufficiently that the cap can be unscrewed. Sometimes a little oil left overnight will loosen the cap. If these more gentle methods fail, take the instrument to a good repairman.
To remove a corroded piston is even more difficult. Don’t try to poke the piston out of the casing with a stick. The piston is hollow, and the walls are thin. This force is liable to stave in the top or bottom of the piston or buckle the sidewall. Put a good penetrating oil in the valve and let it sit overnight. Then try pulling and pushing the piston with a slight rotating motion. If this does not bring success, see your repairman.
Before putting an instrument away for any length of time, apply a little vaseline or tallow to the threads of the valve caps, also a little valve oil on the piston. This will help avoid corroded caps and pistons and will help preserve your instrument. You will have to clean the oil off after storage