Toto uses several different fill valves. If you have the one made for them by Korky, stop at Lowes or a local plumbing supply shop for a replacement cap (around $3), and in about a minute, you will have replaced the only seal in the thing. If it is not the one by Korky, then you have to replace the whole thing, as far as I know. You might want to check the static incoming water pressure. If it is excessivly high, you may need a PRV and expansion tank. A toilet fill valve seems to often be one of the weakest links, and leaks first under high pressure situations. If you are lucky, you could just remove the cap, check if there is any sand or other thing preventing the seal - clean it off, then replace it and solve your problem.