Three things come to my mind here (other than your gauge is correct when at the centre).
Firstly, if the gauge suddenly moved after a hard breaking event, then it is likely a loose wire issue, either at the sensor, or in the dash.
Secondly, naughty you for not checking your coolant level often enough and allowing the main tank to get low ! These vans do not respond well to low coolant levels
Do check it more often and if it needs constant topping up, find out why and fix it !
Thirdly, so your low coolant sensor is not working then. If the main tank gets anything below about 2" from the correct "full to the brim" then the red coolant LED on the dash should start to flash (as a warning to STOP and check what is going on). To check it's function, simply turn the ignition on, don't start the engine, the LED should flash for about 4 seconds and then go out. Now go to the main coolant tank and pull off the small electrical plug that is at the top of it. Now watch that LED. After a delay of up to 30 seconds it should start to flash. It won't stop (even if you replace the connector on the tank) until you turn the ignition off and then back on to "re-set" the circuit. If it fails this test, report back as imho, a low coolant warning light is essential on these vans. (anyone reading this = check yours next time you have the lid off
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