The Stovebolt Site Forums The Shop Area Engine Shop Power Steering Box

You have a couple of things going on here. You indicated that your 65 had a power steering box on it. To me that means that it has been converted to a later steering system (probably 67 up). It does not use a ram.
The 65 had a ram and a control valve which supplemented the stock manual gear box similar to what Chevy passenger cars used from 55 through 64. Fords also used this system. Generally it operated on less pressure than the later Saginaw self contained units which operate at about 1300psi. PS systems are supposed to be self bleeding but like most things they don't always cooperate. Starting the engine before bleeding the air from the system can lead to cavitation which introduces more air into the system. Subsequently you get lots of bubbles and hydraulic systems won't work on bubbles. As 3B also mentioned dirt or corosion could have entered the system which will plug it up.
If you have the ram/control valve system, the pump may have too much pressure and that may be causing your problem.
Fred