I do agree With Mr. Anderson, and his answer is really the best you can do an acid bath and an overhaul are the best ways to make an old horn work like new, BUT for those of us who DON'T have money pouring out of our ears
This is by no means a perfect solution, but if you can't afford an acid bath try this, it has worked well for me in the past:
before you begin, do NOT do this without this necessary equipment onhand:
Slide Grease
Valve Oil
step 1 label a piece of paper with the numbers 1-3
step 2 remove valves and place on over corresponding number label
step 3 remove tuning slides
step 4 make up a tub or sink (whichever appropriate for your instrument size) of water just barely warm to touch (NOT HOT... I CAN NOT STRESS THIS ENOUGH... HOT WILL STRIP YOUR HORN OF ITS LACQUER... DON'T!!!)
step 5 add a liberal amount of Dishsoap (Palmolive works quite well), then get some suds built
step 6 submerse horn and slides
Step 7 go to bed, play games, leave for about 5-8 hours
Step 8 remove from water (CAREFUL, they will be SLIPPERY), rinse (with trumpet best way is to run water through bell), then running water through each loose piece of pipe
step 9 allow all parts to AIR DRY... make sure it is totally dry before moving on... tilt and shake to free water from bends
step 10 grease slides and reassemble, NO VALVES yet
Step 11 oil valves (don't be stingy, slather it on), then reinsert into appropriate casings, make sure they are moving smoothly... if not, MORE oil, the water.