Here is another thought. There were different oil pans and oil pickup units for different 235 engines. It is possible the pan in your engine may have been replaced with one from a different year leaving the original pump and pickup in place. If this is the case, I think there is a possibility the combination might not be compatible and therefore not working well. Take a look at this image:
http://s1154.photobucket.com/user/JonGoodman/media/oilpans19561959.jpg.html?filters[user]=125067356&filters[recent]=1&sort=1&o=0 You can't appreciate it from the angle, however the black pan (1959) has a reservoir which sits lower to the ground than the 1956 (red) pan and the volume and shape of the two reservoirs is completely different. There may be other styles I haven't seen yet also. At any rate, I'd want to measure the pickup height and the depth of the pan to see what I had. Good luck.
Jon
