It ok I've worked it out, the heater vac line was T'ed off the same line as the BBM on the manifold side so it was getting vacuum. The check valve does work as the heater vac line holds vacuum with this in place, so no leaks I presume. Tomorrow I am going to connect the heater line up to the 10mm booster line and see if that maintains a better supply of vacuum to the heater controls.