When you are not towing a trailer, why should you lock the tractor glad hands to each other (or dummy couplers)?

Some vehicles have \"dead end\" or dummy couplers to which the hoses may be attached when they are not in use. This will prevent water and dirt from getting into the coupler and the air lines. Use the dummy couplers when the air lines are not connected to a trailer. If there are no dummy couplers, the glad hands can sometimes be locked together (depending on the couplings). It is very important to keep the air supply clean. [California Commercial Driver Handbook]