Case 580 Super L intermittent reverse problem

I have a Case 580 upper L Series 1. Everything was working fine till a couple of weeks ago when once in a while it wont reverse. Forward always works fine - when I put it in reverse there is nothing - not like it is slipping - just like it does not shift at all. If I uses the machine for 2 hours it may happen once or twice - sometimes not at all. When it does happen sometimes it starts to work after a few minutes - other times I have to shut it off an let is sit for a while. It happens more often when first started but also happens when warmed up and working but not as often.

I have heard that there are snap rings in the control valve that are prone to breakage and that could be the problem - can someone tell me how to go about checking them and what to look for - pictures would be great.

If not the snap rings what else should I look at ?

thanks
Gary

Case 580 Super L intermittent reverse problem