New York Post publisher Rupert Murdock began the 55-day newspaper strike as the tough-talking pointman for the Publishers Association of New York City, but his fellow publishers discovered yesterday morning that their pointman had negotiated a separate peace.

"We are caught in a predicament," an executive of one paper said yesterday. "It will be interesting to see what we do."

Murdock handed The New York Times and the DAILY News a hot potato when he reached a tentative agreement with the striking pressmen in a single eight-hour bargaining session Sunday.

The agreement, according to sources, is as much a device to pressure The Times and The News as it is a contract to get the Post back on the streets.

Murdock has reportedly agreed that if the pressmen return to work now, they will receive from him most of the contract benefits they eventually negotiate with Murcoch's fellow publishers.

However, the pressmen also reportedly agreed to take into account in their settlement with Murcoch that the Post has been losing large amounts of money. Losses of about $10 million were reported in 1977.

As of late yesterday, The Times, the News and the officers of the publishers association has not seen a copy of the Murdock-pressmen agreement.

Murdoch is president of the publishers association. An executive of another paper declined to discuss the deal, saying, "We cannot get from our president the terms of this agreement."

William Kennedy, president of the pressmen's union, said he would not disclose the terms until this afternoon when he puts them to his membership for a vote. However, Kennedy made no secret throughout the strike of his desire to bargain separately with each publisher rather than jointly with the publishers association.

Murdock gave him that opportunity for the first time and he appeared delighted.

It remained unclear, however, what would happen on the central issue of the strike - manning. The publishers have insisted that they must reduce the number of pressmen and take control of the scheduling of pressmen away from the union.

Kennedy said the new contract "projects the union concept of manning." Under the last publishers' proposal before Murdoch broke away from the joint talks, about 200 of the 1,600 pressmen working at the three papers would have lost their jobs.

ALthough a Post spokesman said early yesterday that the afternoon paper could resume publishing tomorrow, Post executives later said it would take them longer to settle their other labor contracts.

The paperhandlers and machinists are on strike against all three papers and the Newspaper Guild is on strike against the Post. Talks between the Post and the Guild are scheduled for today.

The three interim papers that have been appearing regularly during the strike and one that has appeared somewhat less regularly were apparently planning to continue publishing until all three regular daily papers are back.

Neither the Times nor the Daily News said yesterday how Murdoch's separate peace affects its plans.

The publishers had hoped throughout the strike that pressure on the pressmen from other unions unwilling to go without pay in support of what the publishers call "featherbedding" in the pressrooms would eventually force the pressmen into concessions.

Instead, the publishers' solidarity cracked first. Murdoch's defection confounded one theory prevalent from the strike's start.

Since the Post had been losing money and Murdoch has two other New York publications - the Village Voice and New York magazine - that gained circulation and advertising during the strike, it had been assumed that Murdoch would be the publisher least anxious to make concessions.

"The Post has been losing money in the millions of dollars," a key Murdoch assistant said as the strike began. "We need significant relief this year."