The decision could delay Apollos plan to buy US-based Cooper in a debt-funded $2.5-billion deal, which is already facing opposition at a factory in China.
The United Steelworkers (USW) had said terms of the agreement, which covers about 2,500 USW members, will be violated if Cooper closes the deal without Apollo entering into a new agreement with the workers at Findlay, Ohio, and Texarkana, Arkansas.
The USW said it looked forward to resuming bargaining with Apollo and Cooper, USW secretary treasurer Stan Johnson said after the arbitrators decision.
Cooper had argued that the current collective bargaining agreements would continue after the merger. It did not respond to an email seeking comment.