I've replaced my vacuum heater valve three time in the last 8  years on my '56 292 y-block Sunliner.  I buy the Dole OEM valve (not cheap) and can only get one or two  years out of them before they begin to weep coolant from the bottom holes.  I thought it might be too high pressure in my cooling system and went to a lower pressure cap.  The last valve I replaced began leaking again today after a little over two years.  These are brand new Dole valves so I don't believe the rubber diaphragm can be aged or brittle for as little as the car is driven.  We usually only drive it less than 2,000 miles per year.  I've rebuilt the vacuum system and really want the heat control to work as original from the dash control or I'd go with a manual shutoff valve.  Any thoughts or similar experience?    Thanks,   Dean Carriveau