Hi , what type of brass ring did you find ? there are all sorts in there from thrust washers , spacing washers ,bearings etc . What does it look like ? Was it out of the overdrive or the main gear box.
It could be simple and a spacing washer has crumbled ,but Does the solenoid engage ? Is the accumulator spring strong enough ?Are the balls for the pump non return valve and the operating valve seated correctly? Unfortunately if you are finding bits inside I feel you will need to remove and strip to find the culprit.