Geberit model 150-403 toilet Water keeps running after flushing

wd40 possibly

wait for the new washer/ valve to settle in
or turn down the inlet water tap a bit

what does this really mean
tank topping up with water and re filling occasionally

in the wrong way or too tight or the wrong part ?

How old? Who installed? Any warranty? I'd follow that route first.
If not warranty I'd try to get the installer to be interested. Perhaps he will have a "red face" an help out.

Take a close look at the Geberit TM seal...mine had air bubbles in it that prevented a flush seal...and heance a slow leak that caused it to fill-up many times a day and night...see my issue here: https://pierrebridge.wixsite.com/fixit.

You need to shorten the flush time of the flush valve.

You may need to change your diaphram in your filler unit

All toilet bowls have a preset water level determined buy the design of the internal s-trap and can not be change. However if the water level in the tank is not as high as it should be this will affect the water put into the bowel as the bowel in refilled by the flush valve and if it is set to stop before it get filled to the proper setting the bowl water level may be low.
A quick test is to see if the water level in the tank is up to the line in the tank most all toilet tanks have a line either stenciled or marked with a indentation in the porcelain. Another way is to mark the level in the bowel with tape or a marker then pour water slowly into the bowel not fast enough to flush it wait to see if it stays higher then the mark you made or settles back to that level.

Shut off main water supply
shut off water value going to your wall mounted toilet.
look under your apartment or the room ajoining the wall unit.
look for hidden wall space above unit

Hi I have exactly the same problem, in fact it could have been me writing this - 3 toliets fine, main bathroom hisses. The Geberit flushing ciphen on the left (the smaller one) has been replaced and there is still a problem. If you look carefully,the water level needs to come up over the lip of an opening on the Geberit part and this causes the valve mechanism to shut off and stop the hissing. It is fine for about 40 minutes and then the hissing starts again if you don't flush it. The reason is that the water level has gone down but there is no obvious signs of where the water is going. It could be that the central flush mechanism is leaking slightly. I am going to replace this part next. If I were you I'd see if you are getting any water leakage into the pan when toilet not in use to see if the water level in the cistern is going down James T

I hate to say it but you may need a pro on this one, if you want I can try to walk you threw it but let me know. [email protected]