Sounds like you are refering to the handle on the manual primer pump. It unscrews and then allows the pump to be pushed up and down to expel air out of the system after maint of filters etc. There is a new one available that does not need to be unscrewed. Use the search feature and you will find out all about the primer pumps new and old. If it is leaking it needs to be replaced, as it will allow air to be drawn into the system.

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1961 190Db retired
1968 220D/8 325,000
1983 300D 164,150