This is a common source of leaks and trying to tighten an old seal can make it worse.
This is a perfect fit for the O.E. copper sealing ring. You only need one but we sell these as a set of two so you will have a spare.
1977 to 1985 Diesel Hand Primer Pump Copper Washers ( Set of 2 )
$2.95
SKU:
DSL-PRIMEPUMPSEAL 
Product Details
This is where this sealing ring is located. On top of the fuel injection lift pump and at the base of the hand primer pump:
