The Toro 800's have a seal on what we call the "GUTS" of the sprinkler head. I would recommend removing the top cap of the head and then remove the guts by pulling it out of the sprinkler body. You will see a black rubbery seal in which the spring sits. If you hold this seal, you can compress this spring and see how it functions. Wash out any debris from between the seal and the sprinkler riser and rinse the guts thoroughly. Before reinstalling, make sure all components are situated correctly and there are no visible deformities to the seal. If this does not work, the seals themselves may be to worn, and replacement of the head is needed.