With a deadline fast approaching, BART labor negotiations proved unsuccessful yet again Saturday as unions continued to threaten to shut down the popular rail line Monday morning — even as management doubled their previous proposal to hike worker pay.

“We have no choice but (to) leave the table and inform the riding public with enough time for them to make other arrangements for Monday,” John Arantes, president of the local Service Employees International Union, said Saturday evening. That left BART management with no one to talk to, and negotiations were scheduled to resume at 11 a.m. on Sunday — if the unions show up.

BART management and two unions representing about 2,300 blue-collar workers will see their labor contracts expire at 11:59 p.m. on Sunday after three months of negotiations. If a deal is not reached beforehand, workers have sworn they will strike starting at 4 a.m. on Monday.

As unions were walking out, BART management was drafting a new proposal: 8 percent wage increases over four years, double their previous offer of 4 percent over four years, but still far shy of the workers’ previous proposal of 23 percent pay increases.

The average BART worker — including management and non-union employees — made $83,157 in gross pay in 2012, up from $80,588 in 2010, according to an analysis of payroll records by this newspaper. An 8 percent wage bump, on top of a 1 percent pay increase already set to kick in Monday, would push the average BART employee’s gross pay to about $90,600 in 2016.

BART spokesman Rick Rice said they also offered “meaningful reductions” to employee pension and health care contributions compared to what agency officials had proposed earlier. Pay, pension contributions and medical costs are the three big issues left, and unions were set to review the latest proposal Saturday night.

Rice said that their latest offer would give workers enough money to see a “net increase” in total compensation even with workers contributing more to their health care and pension plans.

Union leaders have said they were willing to budge on the issues of pensions and health care benefits but it’s unclear how far they’re willing to go. Currently, BART workers don’t pay toward their pensions and give $92 a month toward their medical benefits, regardless of the number of dependents. Including pay and benefits, the average cost for BART for each employee was $116,309 last year, up from $110,017 in 2010.

“This is a fair and sustainable contract offer,” Rice said. “We’re hoping we’re going to have more conversations” Sunday.

Leaders from the local Amalgamated Transit Union, the other major union, said on Saturday morning that they were headed to BART headquarters in Oakland “with their suitcases packed, ready to stay until a new contract is negotiated and avert a strike.” But that optimism did not last long, and the workers packed up and left at 4 p.m.

Gov. Jerry Brown has thus far declined to weigh in on a union request to delay a strike 60 days. That would give both sides an extra two months to hammer out a pact — but if unsuccessful, it would place the strike during Labor Day weekend, when the Bay Bridge is currently scheduled to close so Caltrans can install the new east span.

A spokesman for Brown again declined to comment on Saturday.

Meanwhile, AC Transit urged commuters Saturday to make alternative plans should its workers also strike Monday morning. The East Bay bus agency will see its labor contract expire on Sunday night and workers have also threatened to walk out if a new deal is not reached before then.

AC Transit officials said they are “hopeful” for a resolution to avert a shutdown and had scheduled a special closed-session Board of Directors meeting for Sunday at 5 p.m. to discuss its labor issues.

Contact Mike Rosenberg at 408-920-5705. Follow him at twitter.com/RosenbergMerc.