One other point to note is that the solid rubber circular seal for the screw top on the reservoir could inhibit the venting of the reservoir by blocking the vent hole. This could possibly cause a back pressure on the reservoir (due to the venting of the larger bore end of the piston via the rear hole in the reservoir base) and push fluid past the pedal end seal when the pedal is pushed. These are merely opinions on my observations and may not be facts in operation.
Another leak source could be the joint between the brass inner sleeve and the machined aluminium block. The joint between the two is under constant pressure at the outlet end when in use and any leakage could track back along the joint between the two metals and appear at the pedal end (as if the rubber seal was leaking. I don't know if there is any sealing between the brass lining and the aluminium block ( dissimilar metals?) other than the pin that holds it in place or if it is just a flush/interference fit?
Again, just trying to come up with suggestions/observations to try and help.
Mike