[Bruder]

That welded tip is normal. Mine has it too. The motor is a stepping motor which means it moves in and out incrementally depending on the sequence the coils are energized. Here is a reference in the online manual that demonstrates how to test. Click here to go there. Note the orientation of the check valve and rubber washer. It lets air in one direction.

 

[Bruder]

To hear that the welded tip is normal make me nervous that I have filed it down to smooth it out and may have damaged it and changed it from what was designed. I hope when we have everything ready to fired it up again it all works out.

I have performed the tests in the manual and could see and hear the plunger moving in and out. It definitely moves a very small amount with every sequence through the terminals.

Don't worry, you just ported and polished it ;-) As long as it doesn't become detached, it should be fine.
The computer control is much faster than you can do it manually. That's why it buzzes and resets when you shut off the engine.