Jocobe, I think that you are misunderstanding the problem. The upper sash is not drifting, It stays up. The problem is that when the window is locked, there is still a gap. Balances have absolutely zero to do with this, as the materials have either shrunk, settled, etc to cause this as has been mentioned already. Bottom line: the windows are due for replacement. To fix it for now (until budget permits replacement): either add a thin piece of wood to the sill or head, or add additional weatherstripping to one or both. You can buy stick-on foam bulb seal in rolls from the store that would probably do the trick for a while. Shimming the lock/keeper would be another option. Ultimately, what you are trying to do is fill in the frame to take up the "slop" that is present when you move the sashes up and down when they are locked together.

I had single hung AL frame windows in a home I sold about a year and a half ago. they were ok, even though the house was a single story ranch going out side to wash the windows was a pain... some wiwdows were harder to reach from the ground and I still needed a ladder, plus some had flower beds outside under them. that made it harder since I didn't want to tramp down the flowers. I am starting to replace the old wood double hung windows with double hung windows that tilt in to clean. I really like them I have to say it makes cleaning them very easy and worth the price to me because I hate to clean windows so much. I have never cleaned the old windows in the house I am in now and doubt if I ever will clean any of the old windows before they get replaced since its such a hassle.

I opened up the sash balance mechanism by grinding the staked keepers that hold the cover in place. Then inserting a putty knife into the housing (had to bend the end cover up for access) and over the spools (to protect them), then a flat screwdriver on top of that knife and turning it to pry the cover free. Not much pressure is required, if you have ground enough of the staking away. I was then able to replace the broken cord with some nylon masons line. I wound the new line around the drum and used it to rewind the spring, repeating until it looked like the other one that was not broken. I then used a small c-clamp to keep it wound and proceeded to place the correct number of wraps of lines on the drum to duplicate the unbroken one (total of 3 ½ turns). Then threaded the line out through the hole in the housing and through the plastic pellet and tied a big knot in it. Then released the c-clamp, while holding the spool with my thumb, and slowly allowing the tension to pull the plastic pellet into place against the housing hole. (I wore work gloves during the rewinding operation so I could safely hold the spool with my thumb when needing to do subsequent rewinding on it. From past experience replacing tape measure tapes, I was careful to watch for and avoid the spring going ballistic on me. It never really threatened to though.) Then I replaced the cover so that the ground away posts were in the opening where they belonged and taped the cover on with USPS packaging tape. :) The cover isn't in danger of coming off since it's held in compression when in the installed position. I wish I had been able to find some heavier line but I think the masons line will likely last well enough. It's rated at 155-pound test. If it doesn't, there is a heavier masons line available that is 216-pound test that I'll replace it with. A tip getting that left side track back into place without breaking it or hurting yourself is to slather some liquid dish soap onto the window stud. It slid right into place easily after I resorted to that method. It also helps to file the sharp end of the spline on the back of the track so it doesn't gouge into the stud. It has taken me nearly as long to post this than to replace the broken sash cord line. I hope it helps someone like me who couldn't find any info on doing this and had to stumble through it. I like fixing things rather than tossing them, especially when it only needs a piece of string replaced. YMMV

I am not sure what a jamb liner is. Back of where the bottom window fits, the wood that comes out between top and bottom windows has aluminum edging. I don't see a name on the hardware or glass. There is no between the glass. This is single paned glass double hung window with pressure fitted screens and storms. The house was built in the fifties.