The Metro board broke a two-week deadlock yesterday and approved a steep fare increase, raising base fares on the Metrorail subway and most bus lines to 75 cents as of April 16.

The board's action means an increase of 10 cents in most base bus fares. Off-peak bus fares in the District of Columbia will remain 5 cents lower than in the suburbs, rising from 60 cents to 70 cents.

The board's action also means a 10-cent increase in base Metrorail fares for all jurisdictions.

In addition, the cost of many rush-hour train rides will rise more than a dime due to higher mileage rates approved yesterday by the board. The Potomac River crossing fee paid by bus riders also will increase by 10 cents.

The board rejected, however, a proposal to charge 5 cents for bus-to-bus transfers, and left unchanged most bus zone fees. In addition, the board made a nonbinding promise not to increase fares further for at least 15 months. The board last raised fares in December 1981.

The new rates reduce fares for one select group of people: the elderly and the handicapped in Maryland. Their special fare will fall a nickel to 25 cents.

The fare package--a compromise ending a protracted deadlock among representatives of the board's member jurisdictions--is projected to generate an additional $14 million a year for Metro, which faces an operating deficit of $180 million this fiscal year.

The new fares are also expected to cause a further decline in Metro's lagging ridership. Rail ridership was down to 266,000 trips per weekday in December, 6,000 below the previous December's figure, but rebounded somewhat in January.

Metro originally intended to implement the fare increases on April 2, two weeks earlier than is now planned. The board members' indecision will cost the system an estimated $500,000 in lost revenues.

Disagreement centered on the rail mileage fee, which goes into effect after three miles of travel at rush hour. Maryland, with many residents who ride long distances, wanted to hold the fee at its current 13 cents. The District, where riders rarely travel more than three miles, wanted to raise it to 15 cents.

Compromise was reached at 14.5 cents. That will increase the cost of a New Carrollton-Metro Center ride from $1.60 to $1.80. Ballston to Metro Center will go from 90 cents to $1.05.

The one dissenting vote in yesterday's decision was cast by Montgomery County representative Cleatus Barnett, who said the rates were "excessive" and would drive large numbers of riders away.

When the Red Line is opened to Shady Grove, tentatively set for late 1984, Montgomery County residents will take the rail system's longest trips and be hard hit by mileage fees. Under the new system, a one-way ride from Shady Grove to Metro Center would cost $2.90.

The District will continue to have a base off-peak Metrobus fare that is 5 cents lower than the fare in the suburbs. The city, with its highly bus-dependent population, has been willing to shoulder the subsidies necessary to keep its fares lower.

A bus ride from Bethesda into the District will now cost $1.35, up from $1.25. A ride from the Springfield Mall area into the District will go from $1.85 to $2.05.

The proposed 5-cent charge for bus-to-bus transfers, rejected by the board yesterday, would have raised about $1.1 million a year. Barnett lobbied heavily for the fee, saying it would limit the number of transfers distributed, thereby cutting illegal exchange of transfers among passengers.

The District, where transfers are most common, viewed the charge as an add-on to the base fare for its riders. Mayor Marion Barry's office agreed to such a charge with the understanding the rail mileage rate would go to 15 cents. The compromise that held it to 14.5 cents eliminated the transfer charge altogether.

The District also gave ground by agreeing to the promise of no more fare increases for 15 months. Facing heavy financial pressure, the city wanted to retain flexibility to raise them before that. The promise is not binding on the board, however.

In a complex series of votes, the board raised fares for elderly and handicapped bus riders in the District from 20 cents to 25 cents. In Virginia, the special fare rose from 30 cents to 35 cents, while in Maryland it was brought down from the current 30 cents to 25 cents.

On Metrorail, elderly and handicapped riders will pay half the rush-hour fare, up to 75 cents.

In approving the package, the board abandoned an officially adopted policy of keeping fare increases at or below the inflation rate. The new fares will increase revenues by about 10 percent, well ahead of inflation in recent months.

In explaining this, Virginia member Joseph Alexander cited fiscal pressures facing the eight local jurisdictions served by Metro and a decision by the board last year to delay the increase from January until April out of fear of worsening ridership problems.