My '70 Mach 1 351C (non air) has a power steering problem. I have a newly rebuilt pump, newly rebuilt cylinder, new control valve assembly and new hoses. All was fine (a few months) until recently when I lost the use of the power steering while driving down the road suddenly. I have full, new fluid and do not see a leak anywhere. As I drive it will work for a few minutes and then quit again. This does not seem to be improving at all and I am at a loss as to the problem. Before taking the entire system apart again, would anyone have any ideas on what may be the problem? Any help you can offer is sincerely appreciated. Thanks for your time. Chris.