A marathon session Saturday failed to produce an agreement, with some of those involved saying the two sides were far apart on some issues.
According to KCBS radio, neither side has provided any media updates Sunday, with just hours to go before the planned walkout.
The unions insist they don't want to strike.
"Our team is giving it our best shot. We really do not want to disrupt service Monday," president of Amalgamated Transit Union Local 1555 told the Associated Press. "We want a deal. We will do whatever it takes."
Unions representing the majority of Bay Area Rapid Transit workers issued a 72-hour notice of intent to strike late Thursday.
BART trains carry an estimated 400,000 passengers each day, and emergency measures such as additional bus and ferry service and increased reliance on casual carpooling did little to cut the sting of a 4 1/2-day strike that severely hampered the region in early July.
With a line of local and regional officials behind him, California Labor Secretary Marty Morgenstern joined San Francisco Mayor Ed Lee in his City Hall office Friday morning to call for an end to the uncertainty.
Lee said that over the last month he had spoken to numerous workers at hotels, coffee shops and restaurants, as well as to owners of small businesses, all of whom conveyed the suffering another strike would cause them.
"I couldn't even get a burrito without confronting someone who asked that we take our stand on behalf of the public," Lee told reporters. "We need an agreement and not a strike in our public transit system. The public ridership … need a voice at this table."
Lee said a strike would hit lower-income families hardest — those trying to get to more than one job who cannot afford the cost of high-priced parking garages. He wanted, he said, "to put a face on the ridership … we need an agreement."
Mediators from the state and federal government have been at the table, but the past month has produced little more than acrimony between the two largest worker unions and BART management.
After making a series of concessions in a down economy in recent years, Service Employees International Union Local 1021 and Amalgamated Transit Union Local 1555 have pressed for raises that exceed 20% over three years.
BART management has countered with a proposed 8% raise over four years. But the agency also has asked workers, who earn a median salary of about $80,000, to contribute to their pensions. BART employees currently pay nothing into the pension system. Workers also are being asked to increase their healthcare contribution from the current flat rate of $92 per month.
Union leaders have lambasted management for being at the table for only about half of the last four weeks because of vacations.