Metro achieved a significant labor breakthrough yesterday when an arbitration panel ruled that the financially pressed transit authority could, for the first time, hire some part-time bus drivers.

At the same time, however, the panel ruled that Metro is obliged to continue paying its drivers and mechanics a full cost-of-living salary increase four times a year. Metro had been seeking a reduction in the increase it is required to pay.

The panel's rulings came on two key issues in the unresolved contract between Metro and Local 689 of the Amalgamated Transit Union, which represents Metro's drivers, subway motormen, mechanics, station attendants and some clerks. The contract expired April 30. Negotiations broke down and the issues went to binding arbitration, as is required by federal law.

The panel's ruling permits Metro to have 10 percent as many part-time drivers as it does full-time drivers. Metro has about 2,800 bus drivers today, so that would mean 280 part-timers. The panel also guaranteed that no existing full-time driver could be laid off for a part-timer, thus assuring that the part-time program will have to begin slowly, as attrition permits.

Members of Local 689 met in Constitution Hall last night to hear the union membership explain the rulings to them.

Metro general manager Theodore C. Lutz estimated yesterday that the part-time ruling "means we could be reducing our operating budget by at least $1 million annually." Part-time drivers, he said, "will give us an effective way to provide rush-hour service."

Under Metro's old contract, all drivers were guaranteed eight-hour days and 40-hour weeks. That means, simplistically, that a single rush-hour bus, even if fullu loaded, could be very costly because its run might last only one hour but the driver would be paid for eight hours.

Metro now is running a combined bus and subway deficit of about $100 million annually. About three-fourths of Metro's operating costs are in salaries.

The contractual prohibition against part-timers has become a big issue, particularly among suburban politicians who are under pressure to add rush-hour-only bus service and keep costs down.

Montgomery County uses part-timers for its Ride-On neighborhood minibus service. Some Fairfax County politicians have talked about pulling out of Metro, operating their own bus system and using part-time employes to do it.

Seattle is the only other big-city bus system that has a contract permitting part-time drivers, according to both labor and transit community experts. Seattle's Metro won the right to hire 400 part-timers last winter after a protracted dispute with its union local.

Joseph S. Wholey, chairman of the Metro board, called the part-time ruling "a major breakthrough." However, he said, "I'm not happy on the cost-of-living decision."

The cost-of-living escalator in Metro's labor contracts has been another sore point with suburban politicians and a strident demand of Metro's drivers.

Under the old contract, every percentage point increase in the quarterly Washington area Consumer Price Index was matched by Metro in a quarterly salary rose in the last two-year contract from $7.26 an hour to $8.16 an hour. Metro estimates that each penny increase costs it $125,000 annually.

The seven-day transit strike in mid-July was triggered largely by Metro's decision to withhold a quarterly cost-of-living increase due July 1 because the contract was still in arbitration. A federal court found that both Metro and Local 689 had dragged their feet on settling the issue and ordered a special arbitration on it.

The special panel ordered Metro to pay the increase under the old contract, which said all benefits would be "undisturbed" while a new contract was in arbitration. Yesterday's weekly paychecks were the first to carry it, and they did so retroactively.L

The new contract, which will run for two years starting last May 1, starts with a base rate of $8.335 an hour for drivers, reflecting the July 1 increase.

Under the new contract, the cost of living will be adjusted on an alternating two-month, four-month schedule instead of quarterly. That is because the Bureau of Labor Statistics has revised its Consumer Price Index for Washington so that changes are reported every two months instead of quarterly (every three months). The wage adjustment will still be made by Metro four times a year. Each adjustment will be a full percentage increase from the base salary over the preceding adjustment.

The arbitration panel disallowed a basic wage increase sought by the union in addition to the cost-of-living escalator.

Part-time drivers must join the union, but will not accrue seniority rights. They will be paid at the full hourly rate, but can work no more than 30 hours a week and must work regularly scheduled assignments only. They will receive a uniform allowance, free rides on Metro and social security, but no vacation or health and pension benefits.

Some 30 lesser issues involving work rules and other matters remain to be arbitrated before the new contract is completed. Both Metro and union leaders agreed yesterday that the major issues are now settled.