The negotiations leading to Montgomery County School Superintendent Charles M. Bernardo's departure seemed hopelessly stalled until the Board of Education made its final offer Thursday. Adding to its earlier offer of 10 extra weeks of salary, the board agreed to provide about $400 a month in pension benefits once he reaches retirement age.

The county school board's offer at a climactic negotiating session with the controversial superintendent and his lawyer resulted in a tentative agreement sparking Thursday's surprise announcement that Bernardo could resign within the next two weeks.

But the compromise accepted by Bernardo came only after six weeks of intense talks in which a series of proposals and counterproposal were presented, leading to the board's final decision to spend between $15,000 and $20,000 for an annuity assuring the superintendent's pension.

The negotiations story began April 1, one day after a county circuit court judge invalidated Bernardo's second four-year contract, scheduled to begin Oct. 1. The board, whose decision to oust Bernardo led to the court ruling, decided to seek an out-of-court settlement to avoid further legal costs that could have resulted from Bernardo's appeal.

The two sides began their talks far apart. According to sources familiar with the negotiations, Bernardo originally asked the board to compensate him for the four years of his second contract at the salary of $53,000 a year.

The board's conservative majority offered to pay him only until he found another job or until Oct. 1, whichever came first. The salary dispute ended when the board agreed to pay Bernardo until Dec. 12 - a salary amounting to $31,200.

Then, the two sides turned to the stubborn question of Bernardo's pension. The pension issue became a sticking point because Bernardo has served as superintendent for four years, and state law requires a minimum of five years employment for retirement benefits.

Finally, two days ago, the board offered to purchase about two-thirds of an annuity that would result in about the same retirement benefits Bernardo would have received if he had served the full five years, according to sources.

Bernardo would provide the remaining third of the annuity payment from funds he has already contributed to the state pension system, which covers county school officials.

According to sources, the annuity will be purchased from a commercial insurance company. Insurance officials in Maryland say a deposit of between $25,000 and $30,000 would be required to assure monthly pension payments of $400 a month when Bernardo reaches the age of 60.

In return for the financial settlement tentatively sealed Thursday, Bernardo agreed to resign and to dismiss the court appeal.

Bernardo announced that he would take immediate leave from his post and that associate Superintendent Dr. J. Edward Andrews Jr. would become acting superintendent.

During the board's executive session April 1 Bernardo attorney Joseph D'Erasmo proposed that Bernardo would leave the superintendency Oct. 1 if the board paid him four years salary and gave him a pension.

The board refused D'Erasmo's first offer outright, according to sources. At the next session D'Erasmo proposed that the board pay Bernardo the equivalent of one year's salary and arrange to pay retirement benefits.

Sources said the board again refused and that D'Erasmo then asked the board to come up with its own offer. Board attorney Roger Titus then offered to pay an annuity and Bernardo's salary through Dec. 12, exactly one year after the board filed its suit.

He also included a caveat: If Bernardo was hired elsewhere before Dec. 12, the board would stop paying him.

At Tuesday's executive session D'Erasmo said "we'll do it," if the board took away the caveat, according to sources.

The Board took the proposal under advisement and began discussing ways to appoint Andrews, a popular 22-year employe of the system.

Sources said board members split along familiar 4-3 lines discussing D'Erasmo's last offer. Conservative Board members Carol Wallace, Eleanor Zappone, Joseph Barse and president Marian Greenblatt felt the board shouldn't continue to pay Bernardo if he found another job.

Board members Elizabeth Spencer, Daryl Shaw and Blair Ewing argued that he should be paid, sources said.

By Thursday's meeting the two sides had agreed to release the contingency, and the board prepared resolutions paying the way for Andrew's ascension.

Andrews will become interim superintendent Oct. 1. He will service until June 1980 when a new superintendent will be chosen by the board to begin a four year term.

According to sources Andrews will be able to continue to work in a "supervisory position" at his present $56,700 salary after a new superintendent takes up the post next year.

Bernardo was unavailable for comment yesterday. Andrews, meanwhile, met with top school officials in his second floor office in what one official termed "get acquainted sessions." The official said Andrews "wanted us to tell him our concerns and help him do the job."