The vacuum stop actuator is a well known weakness. I had to replace mine after only 300.000 miles
This will affect the engines ability to stop, but not the heating system.
Try sucking on the vacuume line to the actuator to find out if it is leaking. Easy job to repair.
Not sure if this one is the problem, but it might be worth checking it.
1. new unit.
2. old unit.
10. new unit in place.