As the MDMs are the brains and intelligence of the ScaleIO cluster, rebuild and rebalance will not continue. When either of those operations need to happen, the SDS nodes check with the Master MDM for permission and where to copy the rebuild/rebalance data. If the MDM is down, there will be no response to those queries and thus those operations cannot happen. 
Do rebalance and rebuild operations work in case of a total mdm loss?
Loss of MDMs