Cal. Education Code § 24615

(a) If the system determines that contributions are due under this part from a retired member, disabled member, or a person who has died, and the person is unable to pay the amount due, the system may withhold all or part of subsequent payments due the retired member, disabled member, or survivor, until the amounts withheld equal the contributions due plus regular interest to the date of payment. Total contributions plus regular interest due shall be recovered by the system within 18 months. (b) Any payment of contributions that a member or beneficiary is required by law to make to the system shall be paid upon receipt of written notice from the system, except as provided in subdivision (c). Payment may be made either in a lump sum or installments, as permitted by the system. Payment of contributions due the system not discovered or unpaid, for whatever reason, prior to the time of retirement, disability, or death shall be paid prior to granting an allowance or benefit to the member or beneficiary unless, in the opinion of the board, the making of the payment prior to receipt of an allowance or benefit would impose an undue hardship, in which case payment may be made by the system withholding not more than 18 consecutive monthly installments from payments due from the system. Those installments may not be less than twenty-five dollars ($25) per month, except for the last installment, which may be less than twenty-five dollars ($25). (c) The mode of notice and the measurement of time within which a redeposit described in subdivision (b) shall be made is subject to Section 22337.