I have no water pressure in the tub, there is plenty of pressure in the rest of the house including the bathroom sinks. I have a two-dial spigot with a pull on the spigot to turn on the shower. There isn't enough pressure to turn on the shower and it even comes out to slow to take a bath. I was told that if could be the antiscald valve. Was I told correctly? Could it be something else? How do I fix it? Do I need to cut a hole in my wall? Thanks.