Drain piping building pressure

Just recently my Cat has started to build pressure in the drain piping while parked in storage. I noticed this the other day when I saw a drip coming from the dump connection. I had drained all tanks and closed the valves after the previous trip. I undid the twist lock and a rush of air and some residue blew out onto the drive way. I am sure it is due to the heat expanding the trapped air but do any of you think this could cause any problems?

With the vents up on the roof, I would think any pressure would be non existent. Unless your vents are clogged.

You should always leave some water in your black tank. A good 3 to 5 gallons. This keeps THINGS floating around, instead of getting hard and stuck to the inside of the tank.

Is it possible that it was just pressure from between the cap and the slide valves?

I was thinking along the same lines as Tom. The vent pipes should relieve any pressure that could build up in the tanks.

With all the valves closed, any pressure in the tanks (caused by a blocked vent pipe) would remain behind the valves and in the tank. Removing the cap from the dump connection would not relieve this pressure unless one of the valves was leaking.

I agree with Tom, that we should be leaving at least 5 gallons of water in the tanks, or just enough anti-freeze to cover the valves and bottom of the tanks during the winter.

I would do a complete sanitzing of the tanks and then see if the problem goes away. If not, I'd check the valve seats which can be a messy job cause you've got to open the slide seats to do it.

Sometimes during assembly, they push the vent pipes too far down into the tanks causing them to virtually seal against the bottom of the tank.

If there was residue in the pipes it would cause gases and pressure.Being the valves were closed,and the heat we have had the past few weeks.I would have go with Tom on the pressure between the valve and closure cap.