Senate-House conferees have approved plans to block until May a long-pending administration proposal to revamp the system used by the government to determine who gets pay raises, promotions or layoff notices.

The conferees also have agreed to language that would give children of federal workers access to low-cost day care services at federal facilities.

Both actions, which are expected to be approved by the full Senate and House as part of the Treasury-Postal Service budget, are the result of a persistent bipartisan effort by members of Congress representing government employes in this area, home to 13 percent of the federal work force.

The ban on proposed changes in federal performance and reduction-in-force rules survived the Senate-House conference committee on the Treasury-Postal Service budget in the face of heavy opposition from the administration, which had sought the changes for three years.

The Office of Personnel Management wants to revamp the rules to give less weight to seniority and more to performance, as measured by annual ratings. Under the OPM plan, for example, longtime employes with mediocre performance ratings could have been laid off before workers with less experience but better performance ratings.

Federal unions oppose changing the seniority system, arguing that current rules give managers enough flexibility to deal with problem employes. They argued that the proposed OPM changes would make it easier for bosses to unfairly punish or fire longtime workers for personal or political reasons.

The conferees preserved language proposed by Sen. Paul Trible (R-Va.) and Rep. Frank Wolf (R-Va.) that would encourage federal agencies to make extra space available, rent free, for day care centers. Agencies now offer the free space to some credit unions and cafeterias.

A few federal agencies, including the Labor Department, now offer day care facilities for employes. But those arrangements were made either by recreation associations or through union contracts. The Senate-House action would make it possible for any agency with space to offer it for day care.