no level restriction
Agree with Tom. no level restriction is NOT a clearance to enter CAS.
Example.
Traffic leaving CAS at 3A against traffic inside CAS at 2A. You leave the zone below an airway which has a base of FL35.
Once outside CAS the Radar service is terminated (possibly any traffic given) and you are perfectly entitled to no longer maintain 3A. The controller advises you no longer need to continue with any level restriction given in your initial clearance.
This is NOT a clearance to enter the airway or turn around and re enter the CAS from which you have departed.