I'm really only familiar with tenth scale Losi and AE shocks, but they all work on the same principles. It sounds to me like the problem is that you're not bleeding them enough - you're leaving too much oil in the shock. If you're only pushing the shock shaft back in 80% of the way before you tighten them down, there is still too much oil in the shock to allow full compression. Thus when you're running the car on the track, and in a turn or off a jump the shock compresses all the way, you put tremendous pressure on the shock seal and it gives out. It sounds like your method for building the shock is fine. Just, next time, try pushing the shaft 97, 98% of the way in before you tighten the shock. After tightening the shocks down (no springs on yet), you should be able to compress them all the way with constant pressure, and the shaft should rebound just slightly when you let go. I'll bet they will last quite a bit longer.
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