Sorry to hear about all of your cooling system troubles.

You came up with the most likely solution when you asked, "Does it need to
be bled?"

It is likely that air is trapped in the system, and air cannot remove heat
from a waterboxer engine the way it can from the aircooled vans.  You must
be sure that the system is full of coolant, not full of air pockets.

There is an involved specific procedure for bleeding the cooling system,
which includes raising the front of the van about 14", running the motor at
about 2500 rpm while pouring coolant mixture into the expansion tank, and
unscrewing the bleeder screw on the radiator until the air has fully
escaped.  This is all covered in the Bentlley manual.  There are also many
discussions of shortcuts and alternate techniques in the Archives.
Do not forget to use only a phosphate-free coolant.

If your mechanic has already properly bled the system, then I defer to the
more technically adept members of the List for further suggestions.

Budd Premack
86 Syncro, 84 GL(for sale soon), 73 Super
Minneapolis, MN (Land of Sky Blue Waters)