Workers in the West Contra Costa Unified School District turned down a tentative labor pact Wednesday, a union leader said.

"We stand ready to resume negotiations," said Richard Leung, supervising business agent of Public Employees Union Local 1, which represents about 1,500 maintenance and clerical workers, paraprofessionals and teachers' assistants in the school district.

The tentative, three-year pact had called for a wage freeze and for members to pay between $133 and $373 a month toward their and their families' medical benefits. The district currently absorbs the entire cost of the benefits, which the union said will be $533 to $1,323 a month in 2010.

Leung did not disclose the margin of the membership vote, which took place between 2 and 5 p.m. Wednesday at Helms Middle School in San Pablo.

The tentative agreement, hammered out by the union and the school district Aug. 18, also provided for an annual reduction of six work days for employees who work 12 months during the year, and five work days for those who work less than 12 months, with corresponding loss of wages.

Leung said the members "simply can't afford" the benefit co-payments.

"Most of our members are low-wage workers," Leung said. "Most make under $25,000 a year."

School district officials could not immediately be reached for comment.