If you have a break in service before you are vested, it is possible for you to lose your Pension Credits and Vesting Service.

Once you reach vested status, you cannot incur a permanent break in service.

A break in service occurs if you are not credited with a minimum number of hours of employment during a specified period of time.
In general, when you have a break in service you are no longer a participant in the Plan. If you have a permanent break in service, the Pension Credits and Vesting Service you earned before the break are cancelled.

However, if you have already earned the right to a pension, which means you are vested, you may leave the coverage of the Plan without losing your pension rights.