I'm sure there is an obvious solution to this but I have a 30l water tank which is connected to an external shurflo pump. What I'm looking for is a one way lid so that it let's air in when the pump is working but doesn't let water out when driving around. At the moment I'm having to unscrew the lid slightly every time I use the pump so there isn't a vacuum created in the tank. I have came across vented lids but these apparently will still let the water out when driving around.