743B Drive Motor, O-Ring breakdown, Disappearing - Hydraulic Oil?

[123ignitionusa]

I recently rebuilt the drive motors in a 743B with in six month ref 21 was missing. On the original rebuild the o-ring was in pieces and partially missing. This time the ring back up was there and unbroken but the o-ring was gone completely. So wondering what could cause this? Type of Hydraulic fluid? Viscosity of fluid? That particular o-ring is a different color than the standard black. Probably a Viton material. Viton should stand up to anything. Would be helpful to many if this is hydraulic related. I use L-HM 46. Any thoughts on this?

 

[123ignitionusa]

What are you guys using for the #21 O-Ring? The green viton o-ring with the LPS kit fails and is unreliable. I bet I don't have 5 hrs on these in the picture.