Those old Continental sidevalves never produced much vacuum, they will run fine even when they get tired and low on compresion. Remove the filterlock and fit an electric solonoid valve filter assy, that will cure the problem, I have done the same myself.
On your question about them old C500's being crap, yes they were a ba**ard to work on but you wont find many Toyotas, Mitsi;s and the like going when they are that old!! We look after a couple of old Clarks for a customer that are 1950's trucks and still going strong.