Let me check to be sure, but if memory serves me and you can check the hose routing, the oil cooler in the bottom tank of the rad is the transmission oil cooler not the hydraulic oil cooler.
The washer you mentioned is a check valve to seperate the high pressure rear pump oil from entering the low pressure front pump oil if it is cracked it will bypass and keep the pump from shifting properly, that could very well be your problem. Try minnpar.com to see if they have it, if not a good machinist could duplicate it, the flat surface is the sealing part. You should try to get the right part that would be ground and heat heat treated. You could try a silver solder repair but it would need to be lapped flat to get a perfect seal.