Harry, if you plan on replacing the booster pump anyway, try this somewhat
cheesy fix first ( worked for me when my vanagon was still diesel).

Remove the hose from the pump and undo the 3 bolts that secure the top
plate of the pump to the body. There is an O-ring between the two parts,
see if it is still good. Examine the top plate, are there circular wear
marks (deep) on the surface? You can polish then out on emery cloth and a
flat surface. Inside the body there is a vaned pump mechanism that will
"screw" up and out when the engine is turned over, pushing itself against
the top plate and making a good seal.
 The black vanes should be free moving in their slots so that they can be
flung outwards by centrifugal/centripetal force when the mechanism rotates.
See if they do move, sometimes as the mechanism wears against the top
plate the slots get sort of peened over and drag on the vanes. I carefully
filed the peened-over slots with a safe-sided little file that originally
is used for sharpening Japanese saws. 

Its worth a try, only takes an hour or so.

Alistair