I had cruise control on my '82 diesel Vanagon. There's a 1 way valve between
the vacuum pump and the vacuum reservoir. The vacuum line for the cruise
control should be connected to the reservoir side of the valve. The symptoms
you explain could be caused by the vacuum line being connected to the pump
side of the valve. There may also be a sensitivity adjustment on the cruise
control. As I recall, the cruise control I installed on mine had a
disclaimer: "not for diesel vehicles" but it worked fine.
Marve
ex '59 Bug
ex '67 Squareback
ex '72 Westphalia Camper with Adventure Camper top
ex '82 diesel Vanagon
'95 Eurobago (For Sale)