They are really, pretty easy to take apart. There is probably some gunk causing the plunger to stick. If it is a clippard, you can take the coil off with the big nut on the outside, then you remove the two screws(or is it four, can't remember) from the top of the valve.
You should then be able to pull the stem off, and you should be able to see the plunger and spring.
Of course, if you are not comfortable taking this apart, definitely contact sumo.