The hot valve is an adjustable valve , done by varying the voltage automatically depending on whether you want hot or warm. The cold valve is just open or shut, no adjustment. The hot valve is dearer as well as it is adjustable. When the hot valves decide not to work there is usually just a trickle from it as you describe.
Unless you get a second hand valve , they are about $75 new for hot and 50 something for cold.
David, July 2007