Your picture is the SFR Repressurization (delivery) pump.
The outlet of the pump supplies SFR water from the “big tank” to the individual bay solenoid valves.
The outlet of each solenoid valve supplies SFR water to the bays.
The hose or tubing from each solenoid valve likely connects to a manifold located above the bay. The manifold connects the SFR low pressure hose to the high pressure hose.
To prevent high pressure
soap/rinse/wax from flowing back down the SFR hose (and eventually into the big water tank , a high pressure check valve must be installed.
So, follow the low pressure line from the SFR solenoid valve to the bay. Where it connects to the high pressure line is where the check valve is likely located.
To troubleshoot:
Disconnect the SFR hoses (one at a time) from the SFR solenoid valve and turn on a high-pressure function for each bay. If water flows out of disconnected hose, that bay’s check valve has failed.