TR;

Derek beat me to it, but that looseness in the mounting holes is what I would point out also...typically, during assembly, I snug the two bolts to allow pivoting, and turn the Slave to optimize this alignment before final tightening (and there's typically a lot of slop to allow this!)...you might try this also...but it might be too late to help now...I expect operating for an extended time with this misalignment and the angle on that Rod with respect to bore axis, has caused the piston to cock in the hole every time it is actuated, and more excessively than normal, see reference link, scoring the bore and that's what's now causing the leak.

If you were to remove and inspect the bore, I expect you would find serious longitudinal scoring in the area which is on top in the angle as shown in the picture. If you confirm this, replacement is the only option as a new seal will likely not be able to deal with bore damage left even after honing...if you replace, I recommend an O'Ringed piston to prevent bore contact, see link.

...and the fact that no manufacturer's info is visible on casting might indicate this is one of those copies of unknown manufacture, in which case the mounting holes may have additional issues contributing to the problem (size/location).

Good Hunting!

Derek; Tnx for reference!