This has been posted before. But look at the
Dexter EZ Lube diagram. You can see that there is a port from the zerk fitting to the rear of the hub. From there the grease flows thru the rear bearing, then thru the hub, to the front bearing, then out the front.
If pressure builds up, the grease would come out thru the front bearing, not out the rear seal or the zerk.
I lubed mine using the Dexter instructions. It did take nearly a tube per axle to get grease to fill the void and flow out the front. But after many miles thru very hot temperature driving conditions, very little grease has come out the front and the brakes still work fine indicating no grease has come out the rear.