low-reverse servo piston breaks

I have a 727 reverse shift pattern manual valve body trans. I have over the years had issues that cause the low-reverse servo piston to break. Case is OK, no excess wear in the bore etc. Other than that it has been one hell of a firm shifting and quick shifting transmission. Has the better planetarys, drums, bands etc. It is in a 64 Dart with a small block. The retainer and springs do not break, just the piston. Maybe I need a better than stock part?

What do you think is causing the breakage, or do I need to go with a billet servo or HD part? Is it just too high of line pressure or do I need a better part here in the trans or something I am missing?his is the only issue I have ever had withthe trans.

Note: I am going to do a complete rebuild because it is do, but while I have it out I want to eliminate this failure.

Thanks!