The lever springs in my 6D have lost their "zip" after 40 years and I was able to buy replacements and replace the one on the thumb lever, but getting the long one through the three levers is giving me difficulty. I released the strings to reduce the tension on the old springs, but it doesn't want to unscrew - any suggestions? There is nothing on-line for spring replacement - which seems odd to me - other than for the water valve.

Getting the long out through the 3 levers OUT is the issue.

My suggestion is to take the thing to a qualified repair person.

The long axle through the levers unscrews from one end and is threaded only at the opposite end. It does take someone with strong fingers to remove the axle, and while I'd waited for a response, my husband managed to loosen it. The axle then slides out - take note of which lever goes with which valve as well the orientation of the springs. I changed out the springs on the levers, threaded them back onto the axle and they are working nicely now. The instrument is pretty simple, mechanically speaking. I don't know why someone would not do the work themselves including recorking the stops and replacing the lever strings. You need a rawhide mallet to disassembled the valves/rotors, but taking them out to be more thoroughly cleaned isn't difficult.

For a soldering issue on a corroded slide, I would definitely take it to a professional, but the French Horn, like other instruments, is designed to not be terribly difficult to maintain. Since no one has written about the replacement of the key springs, now I've done it myself. I take my instruments to a repair person for repairs, not maintenance, but some people aren't handy or don't want to take the time to learn how to maintain things. Sending it to a repair person is better than not doing the maintenance at all.

Too many questions seem to be answered "don't do it yourself" - it smells like collusion or quashing by repair people.

Thanks for removing some of the 'mystique' (or should I say, bovine excrement) from this subject. For removing the valve rotor, having removed string etc and top screw, a 35mm long,20mm diam dowel with a 10mm bored hole (about 10mm deep) at one end is quite enough to tap out the valve. Expensive mallet not needed, in fact, the smaller the hammer the better.
Rotor can be cleaned, gently, with brass cleaner. Clean well with oil and lint free cloth , then re-oil before replacing.
Make sure that the notch on rotor is aligned with notch on cylinder when replacing.

However, if you have any doubts about your practical abilities - don't!!!

Actually, the Conn 1978 has a rotor stem that is keyed to the piece on the bottom - if you hold that in place between the stops, the valve is positioned close enough when you insert it, then you just fine-tune the position after re-attaching the string to the lever. The rotor top I do my best to align, but it's a uniform piece, so I'm not sure why being 90-degrees or 180-degrees off would matter in terms of the top. If I put the rotor in that far off, the bottom won't end up between the stops. It's the relationship between the rotor and the bottom piece that align it for play. I have found that having the wrong rotor top on the wrong rotor position will impact how things adjust, though - at least on the 1978.

Lever springs are working great, though! I wound the thumb spring one turn tighter, but the other levers were improved immensely just by replacing the springs. Mozart-speed once again!