</font><font color="blue" class="small">( My 284 leaks when full to the top too, I never looked, I thought it must be an overflow. )</font>
It's just the sending unit gasket. The OE is little more than a paper gasket, portions of which gradually dissolve away. That "full tank" leak will only get worse.
If you have the same cowl as I had, you can fortunately remove the sending unit without removing the cowl. I used a screwdriver bit on a quarter inch drive ratchet, and fuel resistant gasket material from NAPA.
+Disconnect battery.
+Label sending unit wires, then disconnect (from the sending unit).
+Remove machine screws around sending unit. Be careful to use a large screwdriver bit, as the machine screw heads are very very soft.
+Remove sending unit. Because of limited clearance, take particular care not to bent the float assembly
+Carefully scrape old gasket off both surfaces. If you're lucky there might be enough left to use as a template. Even if not, you still don't want any scrapings to fall into the tank.
+Cut new gasket
+Install new gasket on the sending unit, making sure holes line up. Trim as necessary.
+Insert sending unit back into the tank, taking care not to bend anything
+Tighten sending unit screws. Remember the heads are very very soft.
+Reconnect wiring
+Reconnect battery
+Top up tank
//greg//