Make sure there is not a screen in addition to a filter. If it turns off with the key, check the fuel shut off solenoid. Should be on injector pump. Unplug wire on top, screw it out, remove rubber plunger, screw back in. Then run the tractor without it and see how it does. You can kill it by snufing the air.
A picture would help, but if there is a banjo fitting at the injector pump end of the hose you may be able to bleed the air out of the fuel line between the filter and injection pump. If you have an electric fuel pump somewhere between tank and injector pump make sure the key is on so the pump runs, then just crack the fitting at the injector end until the air is gone, then snug it back up. I usually have to do this whenever I change my fuel filter.
If you didn't fix your machine yet, I have a WM 55 also and it had/has a similar issue. In my case it was poor connetion at the fuel control module and the fuel shutoff solenoid would just shut off whenever it wanted to, sometimes almost right away and other times after a long period of running. Contacts looked a bit corroded. I cleaned the contacts and put some dielectric grease on the connections but that didn't quite fix it. I took the bracket off the firewall so I could get at the fuel control module and started the tractor and then by flexing the connector one way or the other I could get the tractor to begin shutting down or keep running. I ended up putting a shim between the connector and the bracket as a temporary fix and then ordered another module.
