What is the place of customary practices when they’re not specifically outlined in a CBA?

Q: “Our union employees are entitled to a bonus for risk in the area of work. Under the CBA, that bonus is stated for the amount of $450.00 per year. For the last 28 years, the employer has been using paying that amount of $450.00 and adding the extra hours to make the computation for each union employee. (That interpretation was made by the accounting department; it is not in the CBA.) The employer wants to pay what is on the CBA only ($450.00) without adding payment for the extra hours. Does past custom between employer and union employees force the employer to continue to act according to the custom that is not in the CBA? Is there a case that discusses this matter?” – Carlos, Florida