Since February, 1975, Sen Edward Brooke (R-Mass.) has used $350 a month from his campaign committee's surplus funds to pay rent for an apartment on Boston's Beacon Hill, according to reports filed with the Federal Election Commission.

A Brooke aide, Roger Woodworth, said recently the senator - who had separated from his wife - stayed in the apartment in 1975 and 1976 when he was in Boston.

Brooke "hand political meetings and . . . had a little office there," Woodworth said. A few blocks away was Brooke's state office in Boston's Kennedy Federal Office Building.

Woodworth said the apartment is now used for Brooke's 1978 re-electior campaign although Brooke continues to sleep in the apartment's bedroom on Boston stopovers.

Use of campaign funds for the apartment, Brooke's press secretary Alexander Ellis said yesterday "was checked out with the Internal Revenue Service, Common Cause and the Massachusetts attorney general's office.

A review of campaign fund records filed with the FEC shows that many senators and House members with surpluses use their political funds for varied purposes.

In February, 1976, $2,045 from Sen. Herman Talmage's surplus campaign fund account went to purchase a 1972 Oldsmobile which, according to an aide, the senator uses when he goes to Georgia.

In October, 1976, Rep. Dan Rostenkowski (D-I11.) paid $2,343 from his campaign fund for a Sony Betamax video-tape machine.

According to Rostenkowski, his staff in Chicago tapes local news oroadcasts during the week so he can review them when he gets home on weekends.

In December, 1976, Sen. John A. Durkin (D-N.H.), used about $2,000, according to an aide, from his surplus campaign funds to pay expenses for a WATS telephone line in his state office. The line, used to take trouble calls from constituents, was so popular it cost Durkin more than his Senate telehpone allowance, so campaign funds made up the deficit.

Members of Congress do not yet get a general purpose expense account so each sets his or her own rules as to how to spend official, personal and political funds, if necessary, to meet day-to-day needs.

In the past, the Internal Revenue Service dealt with each member's situation separately. Campaign funds used for either personal or official business were included in tax returns as income and, where appropriate, charged off as business expenses.

The only general rule was the recent one set by the FEC - campaign funds had to be used "for legal purposes."

House and Senate ethics committes have up to now paid scant attention to how members spent campaign surpluses.

That attitude is changing.

The IRS has under consideration proposed regulations that would spell out the separation of political and personal expenitures by a campaign committee. By law, a committee can only use its political funds for "influencing the selection, nomination, election or appointment of any individual" to a federal, state or local office.

This year, IRS had a new tax return for political organizations which asked for a report on expenses that were not directly related to campaign purposes or fund-raising.

In the Senate and the House, unofficial office accounts, which paid for many out-of-pocket expenses, have been eliminated by new ehtics codes. And in both bodies, there is pressure to draft tight limitations that would prevent campaign funds from being used for official purposes or for personal expenses.

The Senate, during the ethics debate last month, voted to have its Appropriations Committee explore whether a new official expense account was needed. At present, for example, a member has an allowance that provides federal has an allowance that provides federal funds to get him back and forth from his home state - but there is no money to pay for travel within the state or for food or housing expenses.

Most memebers, like Brooke, turn to campaign funds to cover whatever they spend on trips back home - even though they are termed official business visits.

Travel expenses are only part of the "business expense" costs that members incur. A look at the uses to which Brook, Talmadge and Rostenkowski put their surplus funds illustrates the situation.

Brooke came out of his last campaign in 1972 with a surplus of over $300,000. In the intervening years it grew to $345,825 by Dec. 31, 1976.

Most of it was invested in certificates of deposit which last year earned the Brooke committee $21,000.

The major committee expenses other than taxes in the past two years, have been for the apartment rent and, in 1975, about $5000 for its furnishings.

Brooks has also spent campaign money for flowers in Boston and Washington totaling over $2000 in 1975 and 1976.

The Brooke committee's report for the first three months of 1977 shows $600 spent on "tickets for the Inaugural Ball and parade," and $967 for flowers.

he fund is also used to make charitable and political contributions. In March, 1976, for example, $1,000 went to the Maria Donovan Memorial Fund at Arvinas Junior College in Newton Mass.

In November 1975M Brooke's fund gave $3,000 to a President Ford committee; early this year $1,000 went to the Massachusetts GOP.

"Flowers sent by Brooke to weddings, funerals and congratulations of all kinds," Woodworth said recently, "are part of the business." By filing detailed public reports on his fund, Brooke is one of the few to disclose the practive.

But, as his aide implied, Booke is not the only legislator who spends his funds for flowers, gifts, and contributions both charitable and political. Traditionally such expenditures have been tools of the trade. Whether they will continue under the new congressional ethics codes is something members of both houses are watching with interest.

By law, a campaign committee's income from contributions and its political expenditures is not taxed.

IRS does, under post-Watergate regulations, require political committees to pay taxes on investment of unused campaign funds and any other nonpolitical income received.

For 1976, the Brooke committee, for example, reported it paid federal taxes of $10,617.

During the Senate ethics debate, Sen. Adlai Stevenson (D-I11.) attempted to limit spending from surplus political funds to direct campaign expenditures. He lost, but the vote was close.

Brooke was a vocal opponent.

Talmadge had a $158,581 surplus at the end of last year.

Like Brooke, Talmadge also makes various contributions from his campaign fund: $1,000 for Jimmy Carter was paid out in June, 1976, and $20 last September went for his share of a gift given departing Senate Majority Leader Mike Mansfield (D-Mont.).

The Talmadge campaign fund paid $385 in 1976 for Raymond Bragg, one of his staff members, to take part in "Leadership Georgia," a prestigious seminor for leaders in the state.

Rostenkowski had a campaign surplus of $54,402 at the end of last year. Along with the videotape machine his fund bought $75.29 in tapes for it.

The Illinois Democrat, who is also chief deputy majority whip of the House, said recently he is so satisfied with the videotape machine "I may buy one for my Washington office."

Rostenkowski also spent hundreds of dollars in campaign funds last year making charitable contributions to churches and other religious groups in Chicago.

He said he usually does that in a campaign year. "IRS has never questioned that in the past," he said, "and I don't deduct it on my taxes."

Rostenkowski and others in Congress now worry about "where you draw the line on what you can and can't spend."

The Senate's special committee on conduct recognized "a gray area . . .[where] certain expenditures will not be eligible for reimbursement by the Senate and at the same time will not be directly related to a campaign for election to public office."

It is for that reason that the committee proposed its study of "funds used to defray the ordinary and necessary expenses of the Senate."

Others, however, want firm lines drawn.

Stevenson for example, in the ethics debate said, "There should be a wall between political and public money. That means expense related to official duties should be funded from official funds, and political contributions should be used solely for political purposes."

A new $5,000 House expense allowance will come into being in January, 1978, to take the place of the old, barred, office "slush" funds.

No one is quite sure just what the money will be used for. But since it will be taxpayers' funds and disclosed to the public, members doubt it will be spent on flowers, food or entertainment.