I have EZ lube axles on my 14' tandem axle (7,000 lb rating) trailer with brakes, and I love them so far, with around 5,000 miles on the trailer. I've seen no indications of grease coming out the back seal and getting on the brakes. There is no front seal, only a rubber plug that pulls out. When you fill the cavity between the bearings with grease it begins to push out through the front bearing, where there's no seal to slow it down... Stop pumping and reinstall the plug...
EDIT: In comparison, there's no way with some systems for you to tell, except for resistance on the gun...
One caution though -- only grease the trailer with EZ lube axles before use, when it is cold, NOT after you've driven it and the grease warms up and gets thinner... then it would be more likely to push by the rear seal...
My 2 cents -- your mileage may vary!