Let me start off by saying that if your horn is in need of a cleaning you should take it to a reputable repair shop and get it professionally cleaned...but that is not always an option.

Even at home I suggest a 3 part cleaning.

Soap and water - de-greaser (leaves film)

Distilled Vinegar - Acid (for lime and scale)

Solvent - Denatured Alcohol or NAPTHA

Firstly, you will need to disassemble the valve completely.

Take a paper towel and label 1,2,3 and so on for as many rotors or pistons as you have. All of the parts to the piston/rotor should go back to the corresponding number. Wear patterns to the parts are important for feel and playability.

Breakdown:

A scrub brush will help knock away any build up. make sure you watch out for what the material is made of. I would suggest fine to medium bristles that will not scratch or mar the metals.

The stem - vastly made of aluminum reacts to acids so this should be cleaned in soap and water.

(make sure you take off stem felt.)

Valve guides - Cleaned in soap and water.

Springs - Vinegar

Pistons/Rotors - Vinegar

Bottom Caps - Vinegar

Start off with soap and water - This will de-grease the parts that have oil build up.

Only use vinegar on parts that can handle acids.

-Leave parts acceptable in acids for 5 minute increments. Take out and clean and if needed put back in for another 5 minutes.

Clean the inner casing with solvents as many times as needed to make sure there is no residue.

Oil and reassemble. Get back to playing!