Bear in mind, line of least resistance. You have a brand new cylinder on one side, so the pressure will take the easy way out on the old cylinder. Brakes are like dominos, one goes then the next, then the.... Replace both cylinders., at least. Also how did you bleed it? Most vehicles of that size have equiliser valves bettween front and back brakes, and absoulete mare's nest to bleed. A good pressure bleeder, I personally have no time for the vacum bleeds, will make life a lot easier. They run from about £15. Check any flexi hoses aren't balloning