Hi Kurt,
A full reading all the time, assuming it was working correctly at one time, means that the sense wire (wire from gauge to sender unit) has a high resistance, or has been removed. Since the sense wire can not be easily removed from the sending unit (it is inserted from the top), I would suggest that you disconnect the green/black wire from the gauge (labeled T). With the ignition on, momentarily short the sense terminal to ground. This should cause the needle to show a zero reading. If so that means the gauge is probably good. A common problem with BE fuel gauges, is that the tank (and the attached sending unit) loses a ground. I always add a jumper between the screws holding the sending unit to the tank and a ground external to the tank (like one of the tank mounting screws). A good write up on gauges for the BE can be found at mgaguru.com. The BE uses a system much like the MGA. To answer your other question, no I have not found the aftermarket sending unit to be very reliable. Buy from Moss, and you get a 2 year warranty.....or have an original one rebuilt....by whom, I do not know. They are out there though!