Assuming you are truly talking about the spray heads, and not the rotor type heads, I would suggest replacing them. You can replace them with a different model, or even a different brand if you'd like. I suggest the Hunter Pro Series spray or the RainBird 1800 series. For as cheap as these heads are, it is often more efficient to just replace the entire assembly. The leak around your seal on the head is most likely either a cracked body or a broken wiper seal. My advice, Replace them both.
Tony Posey
Ridge Run Landscapes