Detroit 6V92 fuel leak

I've had the same problem on two different boats, both of them having a fuel tank level physically below the height of the primary Racor filters. One was a Cummins engine, and the other was a Northern Lights generator. The fix for both was to replace both upper "O" rings on the Racors. The previous operator didn't change the "O" rings when replacing filters because they "looked good". The engines would start easily and run fine after having been run within hours or a day, but would require a lot of cranking to get them going after days of non-operation.

If you have Racor filters fitted than there will be a check valve on them in the form of a seated ball bearing type check valve. I've had two seperate occasions where the ball o-ring seat had deteriorated from being old and another time where microbe slime had built up on the check orifice allowing fuel to return to the day tank. Racor sells a rebuild kit with all the needed O-rings, check ball and seals for all of their products.

Isolate or valve off the inlet side of your Racor as Pascal recommended and see if your problem is eliminated. If so its either the flat lid ring seals or T handle O-ring per post #9 or your internal Racor check valve. The rebuild is a simple job but it can be messy.

I would first drain the Racor unit, then disassemble it clean out with fresh diesel get all the dirt and stuff with in the unit out , including the steel ball ,etc. like in the picture above. Make sure you put everthing back together as it came apart! Take a photo or two before and during the disassemble to refer to if you need. And install all new O-rings and gaskets.
While the Racor is apart or before it's apart shut off the fuel ball valve before the filter to stop the flow of fuel. You should have a natural flow of fuel coming from your tank to the Racor . Your engine height doesn't mean a thing, fuel should be freely flowing to the Racor.

If it's not flowing your fuel line is plugged up a little or a lot with tank skank. You may have to blow some air with a small compressor back into the tank through the fuel line from the ball valve back to the tank, to remove the blockage. One or two puffs of air through the fuel line and listen for the air bubbles in the tank to make sure you are through and clear.
Now fuel should come out of the line freely.
I would recommend you get the tank cleaned/fuel polished too if you find the line was plugged up.

See if that doesn't help with your starting problems. I'm not aware of fuel every flowing back to the tanks on boats like yours and mine. When you change the Racor filter it should refill with fuel on it's own from the tank .

Before you start the problem engine, pop off the Racor top and see if there is an air gap with in the housing . Fuel should be up to the top when you open it up. If not that's a problem, it could be the top O-ring or Cover plate gasket it's self as mentioned by others, causing this.

I'm assuming the butyl O-rings on the T handles are spares when changing elements or were they forgot when reassembly took place? If so they're now stretched beyond the threaded stud size and will be a sloppy fit.