Head Vacuum pump runs and waste pressure

[4Chin81]

Hello all,

Several times now when we return to the boat after a week or so away, the pump for the head begins running as soon as we turn the batteries on. It continues to run and will not stop. It also builds pressure in the waste system and when we take it to pump out it explodes out the waste pump hole. When we take the cap off it just spews waste. I have begun unhooking one of the wires to the pump to get it to stop. We rarely use the toilet so it is definitely not full. We also have it pumped out every weekend regardless of use.
We checked the toilet seal and nothing appears to be leaking...we flush and try to make sure that it is not leaking and is setting properly.
So that happened when we arrived on the boat Friday. I hooked the pump back up on Saturday and it pumped for a minute or so then stopped and we had no problem again all weekend. ???
Any thoughts on this would be greatly appreciated. This is the third time this has happened...and I am very anxious to end the waste spewing out of the waste hole!!!

[forbes210]

It looks that one of the problems is clogged waste tank vent. If you or anybody else overfilled the tank it could leave some waste in the vent and after drying out it could get sealed completely. The waste tank shouldn't be under pressure in any situation. At some point you may damage the tank especially during the pump out when sucking the waste out. Running pump is separate issue and its most likely caused by electrical short.

[4Chin81]

Great advice Forbes. That's where I'll start. Thanks.

[Phillbo]

I always shut down my system and flush it to relieve any pressure before I pump out.

Like said above it could be a clogged vent. Can you hear air rushing in to the vent as you pump out? That's how I can tell mine is empty. The rushing air noise gets real loud.

[4Chin81]

May be elementary question but how do I know if the waste tank vent is clogged and how would I clean it out?