APWU reported in 2005:

With the intent being to fix an inequity in how retirement annuities are calculated for part-time postal and federal employees, a federal law was changed in 1986. In instituting the change to solve a problem, however, Congress wound up creating, for many older workers, a new problem. Many postal workers found that their retirement benefits were drastically reduced because they finished out their careers by working part-time.

The way it has worked out is that employees who convert from a long career as a full-timer to a few years as a part-timer get roughly the same retirement annuity as those who work part-time their entire careers.

Fast forward to 2006 Govexec.com reports:

President Bush’s proposal to ease the transition to part-time work for federal employees near the end of their career got lost in the shuffle in the 109th Congress.

Last February, the Office of Personnel Management announced it would ask lawmakers to remove financial penalties on the pensions of employees in the Civil Service Retirement System who switch to part-time hours

OPM transmitted the proposal, called the Federal Retirement Improvements Act, to Congress as part of its fiscal 2007 budget. According to an OPM spokesman, Rep. Jon Porter, R-Nev., who chairs the House Government Reform Subcommittee on the Federal Workforce and Agency Organization, was considering including the fix in a larger federal employee bill. But time ran out and no such bill was ever introduced.