Hello,

This is my first post, so please exuse me if there are any errors,

This very same thing happened to me on my 240D, the problem is
that the center nut, which holds the piston down, was not adjusted properly. The MB manual calls for a special tool to adjust
the roller bearing and spring on the back. Since I did not have that tool, I bolted the pump back on the motor (leaving the front plate off) so I could see the piston move back and forth. On the front of the pump you will see a beveled mark. Turn engine over several times by hand and adjust center nut until the top of the piston meets the bottom of the bevel....

hope this makes sense