How hard should it be to insert a front brake piston? (1974 Kz400)

Hello,

I'm still working on my 1974 Kz400, and am getting real good at
attaching, bleeding and removing my front brake while watching the
kids (I don't let the two year old help).

The piston used to be stuck, so I pumped it out, wet sanded the
piston, put it back in, and it got stuck again. I repeated this a few
times, and now it goes in, when lubricated, very grudgingly. Should I
take it out again and resand the piston and cylinder? Should it be
able to slide in with gentle pressure, or is that too loose?

The oil seal and dust seal still look good, but at 29 years old, I
should probably replace them. If I can't find OEM parts or equivalent,
I hear that some Dodge brakes have similar sized ones, can I just use
those?

Also, where's the best place to get stainless brake lines, should I
just measure how long I need and go with a generic kit?

Thanks,

George