Here is a description of the operation of the control circuits of the Gaggia TS, in case you think you have a fault.
When you first turn on the Gaggia TS, the level control unit flashes the red level light on the front panel for a fraction of a second.
It then opens the fill solenoid and looks for the presence of water in the rear tank. If it is empty, then after 1 second, it closes the solenoid valve and lights the red fill lamp on the front panel.
If there is water in the tank, it keeps the solenoid valve open and starts the pump. The pump draws water from the tank, through the pump, and out through the Over Pressure Valve (OPV). The OPV and the hose route have been slightly modified in this picture.
The OPV remains closed, so all the water flows out of the side of the OPV to the brass tee-piece (with the red cover), which connects to the boiler water inlet via a solenoid valve one way, and to the brew head via a heat-exchanger in the boiler the other way.
As long as the solenoid valve is open, the water will prefer to flow into the boiler to fill it up. The other direction, through the heat exchanger and to the brewhead is higher and so the water does not flow up this way.
The water enters the boiler and displaces the air out of the system via the anti-vacuum valve.
When the level is sufficient to reach the water level probe in the boiler, the control unit switches off the pump and closes the solenoid valve.