The old style ones you usually have to unscrew, pull towards you, push down to pump, pull back up, push down to pump...etc...
Replacing them can be a PAIN. It took me 3 hours, a dremel, brake cleaner, and a vice grips wrench to get it off our 300D. I finally tried spraying the base (all that was left at that point) over and over with shots of brake cleaner and blowing on it to evaporate it quickly and thus supercool the metal of the primer pump. Well, it worked, I grabbed it with the wrench after doing thatm, turned, and *pop*, it came loose quite easily. Ahh, the joys of thermodynamics.
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