It was 20 years odd ago but still got the engine which still runs and the oil flow is fine, there was a slight black ring on the ball, but spring and seat were fine. Not enough to suggest anything but after the 3rd complete strip down I could find nothing else to change so went for it and it cured the problem. Its in the same position as on an A10, after the pump but only accessible by splitting the crankcases and just under the timing side bush. It was the oil flowing and then reversing with mainly air coming back out that suggested to me there was a blockage, without seeing it I would not have believed it as the positive displacement pump should in theory have built up pressure enough to clear any blockage. There is a relief valve and I did not change that but did clean it up, when you look at a gear pump on BSA's they all are made up from 3 parts and the A65 ones especially can leak out oil between the joints, so it equally could allow air in.