Over time this could lead to reducing the life of the valves wiring (as you stated the wires are already brittle), and if the wires are starting to become brittle, its only a matter of time before they corrode and snap apart. The valves are waterproof, obiviously, because of their location and use, and the wire connections are waterproof if you used the proper splices, but prolonged exposure to standing water over time can hurt the wire connections, or the wire itself. If you want to try and remedy the problem, I would suggest digging out an area around the valve, removing the clay, and add some pea gravel to the bottom of the valve box. Depending upon how much water you have in this spot, that should help drain some of the water away without causing too much distruption and spending a lot of money. Of course there are other alternatives, but they are far more expensive. Hope this helps.
Tony Posey
Ridge Run Landscapes