Well for all you young guys I'll straighten out some inaccuracies and maybe give some insight as to why fixing this "check valve" made such a difference.
First, no EGR for years from the date of this guy. Second, sorry Larry, that diesel does have a throttle and a pneumatic governor; which gives the reason why a manifold leak affects running on that guy and not later models (77 on - no more throttles).
I presume the check valve was the one in the line to the power booster. A hole there would alter the manifold vacuum that works on a diaphram in the back of the injection pump. This directly moves the pump rack.