At the site below is a parts breakdown of 2 types of fuel Injection Pumps
The one in the first diagram is the one thay you most likly have.
In the Upper left corner of the diagram you will see.
#29 The Pressure Valve/Delivery Valve Holder
#41 The Pressure Valve/Delivery Valve Spring
#50 The Pressure Valve/Delivery Valve itself (this is a 2 piece assembly the large cylinder of the valve body and in the center the valve itself, the part that contacts the spring)
#47 The Pressure Valve Seal (also called a Copper Crush Washer)
So in the case of the 1 diagram the Copper Crush Washer goes under the Pressure Valve and is no where near the spring.
Do not touch anything further down below #47 and do not loosen the 13mm nuts on each side of the Pressure Valve/Delivery Valve Holder
In the 2nd diagram is an M style Fuel Injection Pump this pump uses both the Copper Crush Washer and the O-ring seals.
The #44 Copper Crush washer in this case goes on top of the #50 Pressure Valve/Delivery Valve.
Here is the site with diagrams
http://www.detali.ru/cat/oem_mb2.asp?TP=1&F=123132&M=617%2E912&GA=722%2E118405&GM=716%2E005++++++++++++++717%2E400&CT=M&cat=143&SID=07&SGR=045&SGN=01