First, I wantto replace it and buy this :
it seems to be the same but it is NOT; the holes are not corresponding ( +/-3 mm difference )
Than I clean the valve ;
to do job , you need :
2 wrench imperial size 3/8 and 5/16
4 bolts M5x40 because the 4 original bolts break.
spray silicone before rebuild
this is the original slide valve :
https://www.valterra.com/product/bladex ... -1-2-bulk/
Notes :
Most of the times, I use my J.C. when windsurfing.
Windsurf means wind +sand+salt water = the worst mixture for the van.
Last session, it was very windy and sand was flying around the van; despite my valve was closed, A lot of sand ( coming from my shower and/or outside ) goes into the valve.
After dismantling, cleaning, rebuild with silicone, exchange rubber seals from inside to outside , everything is now ok with a very smooth movement.
.........ausser das der Abwassertank leer ist 
