Toilet problem filling up when not used

[bernard pilkington]

I have a thetford toilet electric flush from the motor home water supply but if you close the trap it starts filling the bowl up it has code 20090721 which I think is the model number the toilet pump is not going on its own so I'm at a loss...

 

[TheBig1]

it is either grit in the valve or the seal has failed. It depends on the model whether you can access it from the cassette door or need to remove the toilet bowl