Solenoids only control the On/Off flow. From TIR's known instances of cause - check the steel seal disc to verify that the rubber is still bonded to the washer on both sides -- the pilot holes must remain open. Check the diaphragm that it returns to semi flat shape when disassembled. If it retains the cup form like when the valve is closed, it needs replaced.
The following checks follow the main cause described for TIR - Supply of fluid to the pump is being restricted. Check the supply line to the pump for collapsing as the pump draws the fluid. Check filters supplying the pump for blockage. Other plumbing that supplies the pump.
