Maybe you've got the wrong weight of oil? http://www.johnny-loco.com/shop/Download/manuals/Manual-Cargo.pdf

Steering Damper
In order to assure the stability of the
cargo cruiser steering Johnny Loco has
used a steering damper. The oil damper
is adjustable, by moving the ball joint to
another position under the box (closer to
the bottom bracket is lighter, further from
the bottom bracket is heavier.) Adjust this
at a Johnny Loco dealer. It is possible that
during the summer and winter a different
position is need with regards to the
viscosity of the oil inside the damper.

If it's made to have the oil changed, that means you can rebuild it; make it better, faster, stronger...$6 million later...

These guys list a $155 damper as a part, maybe try bakfiets.nl and see if they have something compatible? Bullitt Cargo Bike | Dutch Cargo Bike - Australia & New Zealand