240D Fuel Pump ?

[DanT]

Hi, I just bought a 1974 240D with 4 speed Standard shift transmission. I took the front 2 fuel lines off two change glow plugs. ( Made it easier to do the job) and now I must have air in the lines. I am hoping someone can tell me what the red cap on the top right of the injection pump is for.? I took it off and there doesn't appear to be fuel in the pump under it. It says OEL on the cap, Which I'm told means Oil in German. Should the pump have fuel in it there ? I have dealt with air in the lines before as I have owned several 300 D's but this one is being a bugger to get going. I think a big part is that the primer pump is leaking and needs replaced or rebuilt. I have never seen this red cap on any other Benz Diesel before and am very curios what it's purpose is. Thanks in advance for any help. I'm trying to figure out how to include an attachment with this post. When I do I will add it.