valve sticking

how do you clean a valve that stcks sometimes? i have been told from a buddy that you can just spray WD40 in it but i dont know.. tired of when i hit the down switch the truck sits all the way down lol.. its only 1 valve that sticks so i know its not the switch itself and it only does it sometimes.. it is the dump that is sticking if that means anything different then a up sticking which i dont think it does

What kind of valve is it? I've just taken them apart and used compressed air.

Take it apart and clean it

As the others have said, take it apart and clean it. Remember wd40 displaceswater which means it dries out seals. Keep wd40 away from rubber!