normally the problem is a bad earth on the sender or the gauge.
unfortunatly you cannot get to the sender with the engine in on a bay so its an engine out job or cut an access hole in the bus :shock:
id look at your wiring and give it a bit of a clean on the contacts, also the fuse's... give them a tickle with some emery.
you could drill a tiny hole in the cap to relive the pressure as the breather pipes may have perished over time causing the vacumn mentioned.
there isnt a quick fix really and it may be that the senders on its way out.
otherwise always carry a jerry can! :wink: