On my 06 PT I have always had problems with the valves not opening or closing, last winter I took both out and cleaned and lubed them real good. First trip of the year and the pos would not open so I manually opened the gray tank with the allen wrench.
I decided to run a test, hooked the Fluke to the lead wires at the valves and hit the button. Well no wonder 7.5-8 volts made up a long set of jumper wires with clips on the ends, connected to the house battery's and then tried to see if the valves would cycle. Worked just fine open and closed about 10 times.
On other side where the switches are opened the access door and took a good look at the wiring, what a joke comes from the 14g feed wire to a about 10' of 20g wire to the switches, cut out that wire and unsoldered them from the switches and replaced with about a foot of 14g wire.
Now they open and close like they are supposed to, so before you go to the expense of new valves you need to check the wiring and see just how much power to getting to the valve motors.
Also I ohm'ed out the switches and they had pretty high resistance through, took the fuses out and cycled the switched back and forth until they actually cleared up, now resistance through them is around 1-2 ohm's.