How do you fix old trumpet?

I'm going to guess that the outer valve body might have a small dent in it.

A well-stocked machine shop might have a set of tools (I forget the name of them) that consists of sets of precise balls that can be used to "stretch" holes - they are forced through a hole via an arbor press or similar. You would not want the entire cylinder to be expanded, just to have them run a ball that is exactly the diameter of the interior of the valve body, or maybe a thousandth or two larger.

Failing that, you could try pushing on the dent with something, but that is risky.

If it is due to a dent, but it's very small, you might be able to "lap" the high spot down using some grinding compound on a wooden dowel or other cylinder shaped object that's close to the diameter of the interior. Or, just wrap some fine abrasive paper like 600 grit wet or dry around a dowel, and wear down the high spot like that.

Keep looking around for music instrument stores. I have a trumpet that is 22 years old. I played it, several cousins and now my daughter.

I has been damaged and neglected a few times and this place makes it as new as possible and the highest bill was just over $200. If you find the right place you can probably get it done for about the same

I'm sorry. I can't tell you how much it would be because I don't know. My sister passed out and fell on her trumpet (boy that was fun). Our school's band director's nephew (I think) goes to U of I and is majoring in stuff like that, so he fixed it for us. Now there's still a semi-large dent, but it doesn't affect playing it. Maybe you could try finding someone who knows someone else to fix it for you for free or cheaper than what it would be?

Home and Garden isn't the best place to ask about repairing a musical instrument......