President Reagan is expected to sign legislation next week that would close a retirement loophole that has provided a benefit to part-time workers at the expense of longtime civil servants.

Under existing law, employes can work part time for most of their careers, then convert to full-time status several years before retiring and take full advantage of government retirement rules.

The rules currently base pensions on years worked -- but not hours -- and on the average income over the individual's three years of highest salary.

That quirk has permitted some workers who spent most of their careers in part-time jobs to retire on the same pensions as full-time employes who had paid much more into the retirement fund.

The retirement change that the president may authorize next week is part of the Consolidated Budget Reconciliation Act of 1985. Despite its name, the bill was cleared by Congress only last month.

The bill calls for prorating retirement benefits based on the number of hours worked, rather than years. It wouldn't change benefits for full-time employes, but would trim pension benefits for part-time workers who switch to full-time status late in their careers.

The prorating system would not affect part-time employes who already have moved into full-time jobs. Injury Rules Change

The American Federation of Government Employees has gone to U.S. District Court here to challenge the Labor Department's right to limit medical payments for employes injured on the job.

As part of a budget-cutting program ordered by Congress on March 1, the department began reducing all medical bills submitted for treatment of workers in its Workers Compensation program by 7.3 percent. The union contends that the department has overstepped its authority.

The Labor Department action affects about 85,000 federal and postal workers who lose time from their jobs every year because of injuries. It also affects their doctors.

The department pays medical benefits for the injured workers. But to make spending cuts mandated by the deficit reduction act, the Labor Department is paying only 92 cents of every dollar charged for medical treatment.

The department also plans to issue rules that would bar doctors who insist on making employes pay the difference from participating in the program. Rehire the Controllers?

The Public Employees Department of the AFL-CIO has endorsed legislation introduced by Rep. Guy V. Molinari (R-N.J.) that would encourage the Federal Aviation Administration to strengthen its air safety program by rehiring some of the air controllers who were fired in 1981 for participating in an illegal strike.

Although some of the more than 10,000 fired controllers have returned to FAA jobs, the agency is reluctant to bring back large numbers. Officials contend that many have lost their skills, and their rehiring could cause problems with employes who stayed on the job.

John Leyden, director of Public Employees Department's federal-postal division -- and former president of the Professional Air Traffic Controllers Organization -- says that the air traffic control system is "seriously understaffed" and morale is way down.

"Former controllers represent a valuable resource of skilled specialists available right now," he said. Leyden had left PATCO before the strike took place.