If it slides down quickly, you have a loose fitting slide and need to apply the proper amount of lubricant. Space Filler is the first step, then possibly moving to thicker oil or Anhydrous Lanolin. If it continues to slide down quickly after using thicker grease, you may have a leaky 3 rd valve. Some simple habits can help preserve the appearance of you horn. Perspiration from the hands contains certain acids, which attack metal.
Some perspiration is alkaline but is just as harmful as acid.
In some persons, the perspiration is so strong that holes are eaten right through plating, lacquer and even brass wherever the hands touch the instrument. Keep your instrument free from perspiration. Wipe the perspiration from your horn. Using a leather valve protector can be helpful but replace the protector periodically because perspiration soaks into and through the material. Saliva from the mouth also contains acids, which attack metal.