Periodically, without waiting for leaks to develop, arrange to have faucet washers and seats replaced.  Insect the ballcock or flapper valve in each toilet tank.  Have spare parts on hand to make quick replacements.  The quickest and best way to make plumbing repairs is before a crisis develops, when you can select a time that’s convenient for you and the tenants.  It’s better to shut down an entire building for one day to replace all worn plumbing parts before they fail, than it is to shut down repeatedly for short intervals to handle emergencies.  Repairs never get cheaper if you wait, but waiting increases the risk of higher costs and greater damage later on.  It also increases tenant dissatisfaction.