There is usually a vacuum switch at the top of the machine with a small bore clear pipe leading down from it to the back of the pump out housing. This gets blocked at the pump housing with soap and stops the switch from working. A test for this is to remove the top of the machine and remove the pipe from the switch. Attach another length of pipe to the switvh and suck on the pipe if you here a click you know the switch is working and that the pipe is probably blocked.
Doberman, October 2005
