had the same pblm on my 69 gal which is pbly nearly identical to the system on your marauder. like coosbay said it was the headlight switch. on my headlight switch, on the underside of the plastic unit behind the pull knob, there is a small plastic 1/8' knob that sticks out. i used to have to reach under and manually push that piece forward or back respectively inorder to completely seal the vacum system.

may not be a cure for your pblm but something to check.