The San Francisco police union is seeking to block the release of information from officers’ personnel files, joining a growing number of California law enforcement agencies and labor groups working to blunt the impact of a new transparency law.

Senate Bill 1421 opens up previously sealed peace officer records if they involve serious uses of force or sustained findings of dishonesty or sexual assault.

At issue is whether the law, which took effect Jan. 1, applies to records created before then. A number of unions representing city police officers and county sheriff’s deputies across the state have filed lawsuits against their respective jurisdictions, arguing the law cannot be applied “retroactively.”

Civil rights organizations like the American Civil Liberties Union, as well as bill author Sen. Nancy Skinner, D-Berkeley, say the law targets all records maintained by city, county and state agencies, regardless of when they were created.

In a lawsuit filed Friday in San Francisco County Superior Court, attorneys for the San Francisco Police Officers Association said they will ask a judge to block the city from releasing records from before Jan. 1.

The Chronicle and other news agencies have requested San Francisco records under SB1421, and officials had planned to begin releasing the files Friday, before agreeing to hold off while the lawsuit is considered. City attorneys maintain the law was intended to unseal all responsive documents, and they agreed to delay the release in exchange for a prompt hearing.

“There’s nothing in this law that limits it only to future records,” said John Coté, a spokesman for City Attorney Dennis Herrera. “We are going to proudly defend the public’s right to transparency in their government.”

San Francisco police union representatives did not respond Monday to requests for comment.

Several other efforts to block the release of pre-2019 records are still making their way through courts, though to date they’ve seen little success.

The California Supreme Court declined to weigh in after the San Bernardino County Sheriff’s Employees’ Benefit Association took the matter to the high court.

A Contra Costa County Superior Court judge last month denied a request by six peace officer unions in the county to prevent the release of records.

However, Judge Charles Treat also ordered the cities and county to refrain from releasing the records to allow time for the unions to appeal his ruling, which they did.

The court of appeals then issued an extension on the restraining order pending further briefings, according to the unions’ attorney, Jacob Kalinski.

A Los Angeles judge also rejected an argument lodged by local police and sheriff’s deputy groups. The police officers’ union chose not to appeal the ruling, and the deputies have not yet filed an appeal, said Melanie Ochoa, an attorney with the ACLU of Southern California.

The Ventura County Deputy Sheriffs’ Association scored a tentative courtroom victory to continue blocking the release of records, but only until a higher court issues a ruling.

“I think it’s clear that the courts are rejecting the police unions’ attempt to delay and impede access to the public for records of police misconduct,” Ochoa said.

But the unions have found a champion in California Attorney General Xavier Becerra, whose office is only releasing records beginning in 2019 while lawsuits work their way through court.

Becerra also threatened Bay Area journalists with prosecution if they refused to destroy a list of officers convicted of crimes, which his office claims was improperly released after a public records request.

The matter of retroactivity could come before the California Supreme Court again if lower courts issue differing rulings, or if unions fight an unfavorable decision by the Court of Appeal.

“I think that, from the perspective of my clients, we would appreciate direction from the Supreme Court,” said Kalinski, who represents several unions throughout the state.

Megan Cassidy is a San Francisco Chronicle staff writer. Email: megan.cassidy@sfchronicle.com Twitter: @meganrcassidy