I often am tossed between tackling something that I know is on the way out (this valve has not been one of those items, yet!) or following the adage of better left alone 'til it breaks! If its a water pump I would replace it right away for example but a non critical item might be left till later.
I like to buy small items (well even a few big ones) on eBay or off wrecked cars when I have the opportunity to get a good part for cheap. I saw a levelling valve on eBay recently and let it pass. I probably will get one and try replacing just the O-rings with commonly available ones as a way to "practice" on one before one of mine starts failing. Then I'll be ahead of the curve, I'll have one on hand. That is sorta my philospophy and the garage is full of stuff to prove it.
1971 220 (gas) 4-spd manual 106441
1979 300TD w/ ’85 turbo engine 296650
1983 300D 243280
1985 300TD 223470
1987 300D 262300

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!

My TD also saggs if left alone for 4+ hours. It also droops some when at rest WITH the engine on. I can let it idle for about 10 minutes then rev it to 2k and it will pop back up an inch.

I finally measured the drop. At the trailer hitch, the car rose up exactly 2 inches immediately. I measured the height of the hitch, started the car and as soon as I got back to the hitch again it was already 2 inches higher.

Without really staring at the side of the car and thinking about the drop, I would never notice any sagging.

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1987 300TDT
1981 VW MKI Caddy 1.6 diesel, waiting on engine swap
1983 D-50 Power Ram 4x4 "Mitsubishi" 2.3 turbo diesel
assorted gas powered crap and motorcycles

RIP: 1984 300TDT, 1982 300TDT, 1984 190D 2.2, 1992 300D 2.5, 1987 300TDT, 1982 Maxima LD28, 1983 Maxima LD28, Isuzu C223 P'ups X3, 1983 Holiday Rambler 6.2 Banks turbo diesel, 1984 Winnebago LeSharo 2.1 TD, 1985 Allegro 6.5