If you have a w123, it is on the injection pump. 2" knob, towards front of the pump. Unscrew it--counter clockwise---and pump up/down. Don't forget to tighten it back down. Then tighten the bolt on top of the filter.

BUT, if yours is anything like mine, it probably doesn't work. You'll know it doesn't if fuel starts squirting out around the stem of the pump. You can buy a replacement pump [aftermarket] or just fill your filter with fuel, install the filter, and crank it, being prepared for stall. I keep mine revving to keep it from stalling because if it does [stall] you'll wear your starter out cranking it. Takes a lot of cranking to pressurize the system again.

Hope this helps...

Cheers,

Bill