Schools must cancel up to 70% of a Perkins Loan if the borrower has served as a Peace Corps or CNCS (under Title I, Part A of the Domestic Volunteer Service Act of 1973) volunteer. An authorized official of the Peace Corps or ACTION program must sign the borrower’s cancellation form to certify the borrower’s service. Schools apply cancellation for volunteer service in the following increments:
15% of the original principal loan amount, plus any interest that accrued during the year, for each of the first and second 12-month periods of service and
20% of the original principal loan amount, plus any interest that accrued during the year, for each of the third and fourth 12-month periods of service.
You can find more information in chapter 6 of the Federal Student Aid Handbook.