The design of the rear upright pivot is such that to seal off weather, both o-rings in the stack must be compressed. So it may not be 'free play' that's causing your trouble. Those rather stiff outer o-rings must be squeezed a bit to get the assembly in between the lower legs but still keep water out. You do NOT want the system to just slip together! If they do, water can get in.
It's also incredibly easy to have the short locating dowel slip out of its hole in the thrust washer; if your washer(s) have a series of small dents in the bronze surface near the pin hole, they came from the washer being mis-registered during assembly. Note the bronze surface on the thick washer should be facing a solid surface; that's the function of the thin washers. Without them, the bronze side of washer should rub on the smooth surface of the upright. When I tear miy uprights down, it takes pry bars and a 12" c-clamp to force the parts back together. ******* grease helps.