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:
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$7.85
Currently out of stock.
19mm USA Locking Capable Crowfoot Wrench
WARNING: This will NOT WORK ON THE BOSCH BRAND PRIMER PUMP!!!!
This is the best tool to use when installing or removing the Monark hand primer pump. PLEASE NOTE. This is 19mm and will NOT work on the Bosch hand primer pump or other style pumps (most others are 17 mm).