There is a shut off valve on the Flushmeter, hiden in the 90 degree ell. Usually, it is covered with either Snap On or Crew In chrome cap. Remove the cap and close water with flat screwdriver.
Now, remove the large 5" cap, there is a vacuum breaker rubber disk and few other parts. Remove them and take them with you to Home Depot. They now carry Sloan repair parts. Or go to more professional store, like Ferguson Enterprises. Also, if you have time, order it on-line for the best price.
BE CAREFUL: there are about 18 different Flushmeter models. In addition, they can be for urinals or toilets and can be either full flush or low flush. Not all are parts can be installed on other valves. Purchase the right part, install in in back order, turn water ON. Now, you should be back in business...
See this link to ID your flushmeter valve: http://www.sloanplumbingparts.com/fl...e-flushometers