Higher Level Pay for Custodians

In the January/February issue of the American Postal Worker and in the Maintenance Article “Winning Automatic Remedies for Staffing,” employees are told to punch in to proper operations and get higher-level pay whenever appropriate (for example, for driving or P.I.E. operations). Our custodians are asking for higher-level pay in traveling to the stations in Postal Vehicles. Management is balking and asking me to show them in the contract where they are obligated to pay the custodians high-level while driving to their duty assignments. Can anyone help me out here?