The central heating system in my 50 year old home seems to be losing water pressure. We thought it might be due to some pipes that were running in a concrete slab and had leaked. We bypassed that, but now the pressure is still not holding. We see no leaks anywhere. Any ideas?

Are you having to add water? Is the system set up to automatically add water? If so shut it off and see if you lose water. If so. You have a leak, and must track it down. Otherwise look at the pump. It may be getting old and the impeller corroded away. Any control valves may not be working correctly. Look over the system and post back with what you find.

What is the correct pressure (psi)? I have a split level, the furnace is in the basement, the top floor begins at 12 feet above that. When I add water the psi goes up to 20. It then drops within 12 hours to 10psi. I believe that air is getting into the system because some of the radiators get cold. When I add water and watch the meter go back to 20psi those radiators work again. If this is due to a water leak, I have no idea where that might be. I don't see any sign of one.

If you are having to add water, it is going somewhere. It could be going out a vent or overflow somewhere. Walk around the house looking for a pipe where water has flowed out. Also check inside for a pipe going from the heating system to a drain. Once you find where the water is going, then you can start trying to figure out how to fix it.

I think I found it. I started opening up radiator covers. I found one (on the top floor) where the pressure valve is leaking water. Looks like it's been doing that for years, it's all rusty. Could this be the whole problem?

You can hope so. Fix the known problem, and observe. I would think a very bad long term leak would have caused other problems. Plaster doesn't stand water very well. If it is just leaking a little around the stem, you may get away with tightening the packing nut.