I'm not sure of those are Rain Bird Valves, however they all work pretty much the same. The large knob is for flow control and for a Rain Bird the knob would be smaller and the bleed valve would be right next to it. It will look like a large plastic screw. However, turning the solenoid about 1/4 turn counter clockwise will give the same results. You may need to use pliers to loosen the solenoid, then only hand tighten.
The cause of a leak or weeping valve is usually the valve needs a rebuild. The neoprene diaphragm can get hard and not close properly, or as you might suspect caused by debris. Caution, do not fully remove the solenoid unless the water source is turn off with a stop valve or the well pump is switched off, otherwise you'll get a face full of water.