I recently started a small collection of cornets (mostly). My two oldest are an A. Hileron from the 1890's and a Lyon & Healy (Own Make) from about 1903. Both have stuck slides. Should I try to get these loosened or should I leave well enough alone? I play each one a little - maybe twice a month. I worry that if I try to get the slides moving, I might end up with a damaged horn. I welcome your input.

Full horn list:
Conn Peck Horn (Pan American, 1927?)
DEG Flugelhorn (Signature, late 1960's?)
A. Hileron Cornet (late 1890's)
King Cleveland Cornet (602, late 1970's)
King Silver Flair Trumpet (Pre UMI, 1971)
Lyon & Healy Cornet (1903)
Martin Imperial Cornet (Official Music Man Model, 1962)
Martin Committee Cornet (Deluxe, 1960)
Mirage Pocket Trumpet (1980's?)
Olds Mellophone (marching French Horn, 1978)
Olds Ambassador Trumpet (1956)
Olds Ambassador Cornet (LA, 1949)
Olds Ambassador Cornet (Fullerton, 1956)
Olds Ambassador Cornet (Fullerton, 1967)
Olds Special Cornet (LA, 1948)
Selmer Melody Maker Cornet (1957?)

I would spray with PB Blaster or any rust remover and allow to soak for hours to days really. Test to see if the slides are free at all. If not, reapply the rust remover on the outside point and straight into the tubing ( leadpipe or thru valve casings with valves removed ). Don't force anything. Use about 10 lbs of pressure to pull the slides. It's a pain and requires patience, but it's worth it to have a fully functional horn. You can always take it to a shop and leave it for a week or so for them to do it.

Yeah, don't force anything. After soaking the joint with something (Kroil is really good, too), you can loop a strong cloth through the slide and give it a sharp, short jerk or two. Don't jerk too hard, or you'll either pull the slide apart, or sling it across the room if it comes free. If this doesn't work and it is really stuck, take it to a shop.

I have found that dirt and grime on the inside of the slide can make them stick. Be sure and brush out the inside of the tubing, if you can, before you pull. You have to be really careful on old horns when you pull. You can break the braces if you pull too hard.

If you haven't cleaned the horns clean them in hot soapy water and let them soak. Rinse them with hot water as well. If the horns are lacquered do not use water that is boiling. I have had several stuck slides loosen up after a hot bath like this. Also play the horns. Sometimes stuck things loosen up after being vibrated. Given the age of these, you don't want to break anything. If they are still stuck, just play with them stuck or take them to a professional repair person. Getting the slides unstuck is relatively inexpensive compared to solder work.

My favorite cheap trick is just to run hot tap water over the outside of the slide for about 30 seconds and it seems to break em loose every time.

You can try heating it up a little with a warm wet towel. That would slightly make the outer part of the slide expand, and it might be easier to pull out.
If it doesn't work, go to a repair shop.

I did all of the above to no avail on a newer horn. Be prepared to spend about $30 or less each for removal. Horns that old , I would take to a tech. Cheaper than a repair>

PB Blaster is the best...but the fumes from it are really bad...
You spray it ...if you can inside that helps (but you won't be able to play it until you clean out the horn)
Use a rawhide mallet ( if you try anything made of metal it will ruin your horn) and light LIGHTLY tap
heat it carefully with a heat gun .. and I mean carefully or you will burn the lacquer and melt the solder joints. A heat gun can literally desolder your horn.
tap again
I have used thick rope to tug at it but you have to be careful and hold the pipe so their isn't any stress put on the braces.
Shops have a piece that is molded to the shape of the pipe. They put it in the middle and tap on it to knock the slide out. If it isn't mild to the shape of the side you will put a nice dent in the slide.
This can take a few weeks if the slides are really stuck .. Yamahas hold the record in my house 10 days for the slides to free up. Doing all this everyday.
Honestly it's only $30 to free up a few slides. So my suggestion is to take it to the shop.

Take it to a tech. the Lyon and Healy is a really nice horn. Scroll down on this web page Completed Projects to find an example of a restored "own make" they say it was quite a player when completed, My husband has one that plays so nice we are having it fully restored including the gold plating where it has worn.