Volvo Hi LI Range Valve Problem
 

I have a 1995 Volvo single axle dump truck (WCA42T cab, 9 speed VT1309 transmission). I cannot get the truck to consistently shift into low range. It wants to stay in hi range. I replaced the O rings on the range inhibitor valve (air operated spool valve) and I could not find the exact match o ring (even from the Volvo dealer ). So I think the valve is a little more sticky than it should be. I am not sure what the purpose of that vent on top of the valve body is for? Is it supposed to allow air to escape? Not sure if this matters but the truck was a single axle tractor before being converted to a dump truck. If anyone can share any experience with this range valve it would be appreciated. Thanks.
Mountain D.