LF wheel cylinder.

1946 K I am working on.
All new brake lines, new M/C, new wheel cylinders.
I cannot get the LF one to stop leaking when I put pressure with the brake pedal
This is the 3rd wheel cylinder I have put on this postition and they have all leaked under pressure.
The orginial 4 were all purchased from NAPA as has the replacements for this postition.
I ordered and used the part numbers that are listed on this site.
This one is the only one giving me problems.
any thoughts or suggestions?
Thank you.

disassemble and make sure cups are in the right way,

I bought 4 from speedway and two had cups in backwards,

flat side of cup faces piston, flare side faces outward.

Is it leaking on the brake line connection or is it leaking out the end of the wheel cylinders?

Out the ends on the wheel cylinder.

I agree with Jon. The cups could be in backwards or the cups are not the correct size.