Management can send whoever they wish home. BUT--the regular needs to be paid for the day, plus EMA, or file the grievance. Also, the RCA needs to be paid for the time he was working "off the clock", or the steward should file the grievance. Also, the RCA is entitled to a minimum of 2 hours that day (which he would get if he was working another route).
If all of the above is not done, contact your steward.