I'm having problems pumping up my (ghetto) tubeless tyres. The valve/threads get seized up with the tubeless goop (Joe's no flats sealant) and it's gotten to the stage where I need to use a little pliers to loosen the valve core, and even then it often won't pump properly as the valve is clogged up. Is the valve shagged or is this just a side effect of having the goop in the tyres?
If you can get the valve out.. leave it to soak in some white spirits or meths or some other solvent... that will dissolve the goo and the valve will be good as new again..
There could also be goo inside your pump head / valve doohickey doodaad... as you remove the pump head from the valve there will be enough back pressure to force some goo back into the pump head.. the same fix applies here.. just get some white spirit in there to dissolve the goo.. then lubricate with WD40 (or similar) to stop it happening again..
you can take the valve apart Marc (screw off the top barrel with a pliers while holding the body of the valve with another pliers) and clean off all the dried goo from the core/stem
I usually use a bit of engine oil or wet weather lube/thick oil on the valve core - helps prevent goo from collecting on it again