Rear of the engine is either the heater core fittings(you hope) or a coolant passage at rear of the head gasket.

If you have been bleeding this much and still have the waterfall, there is a leak. I bleed ONCE and never go back unless there is a leak.

Get under the car and look at the back of the heads and below.

 

borrow or rent a pressure tester so that you can find your leak, there are also core plugs in the rear of the heads. iirc

bring your system up to 20 psi and see if it will hold it for an hour or so