... that the piston and seal fit into. The seal never makes contact with the outer tube. But the back of the piston probably will, so anything that stands proud of the inner surface of that tube will have to be knocked down so that it doesn't gouge the piston. But there's no need to worry about sealing, which is a different kettle o' fish.
Bronze brazing would certainly work, but that involves putting a lot of heat into the tube. Not sure how that'd effect the parkerized finish.