Other than replacing your van's filters, it could be that the system is drawing air somewhere along the line, as most diesels are, nowadays, self priming, there could be a flexible pipe that is drawing air into the system, which is more apparent when the system is stressed. If you can bleed the system, check whether there is air being pumped out, which would indicate that it is sucking air, somewhere. Hope this helps.
davidj, August 2011
