Transportation secretary Brock Adams said yesterday that he will order an agency in his department to begin enforcing one-half of a truck safety rule adopted by another DOT agency.

The controversial rule requires new tractors and trailers to be equipped with brakes that keep the wheels from locking during an emergency stop. The rule has been effective since 1975 on new equipment.

It was adopted by the National Highway Traffic Safety Administration, but has not been enforced on over-the-road equipment inspected by the Bureau of Motor Carrier Safety (BMCS), a part of the Federal Highway Administation.

Truckers oppose the rule, charging that antilock braking systems are not reliable and have caused accidents instead of preventing them.

Adams said yesterday a new rule to be proposed "shortly" will required the BMCS to enforce maintenance of the antilock system on tractors but place a moratorium on the requirement for trailers.

Adam's new proposal is in line with a fallback position adopted by the American Trucking Associations in a petition to the Department of Transporatation field Jan. 23.

In that petition, the truckers asked for a full moratorium on the antilock systems, but suggested that "a partial moratorium would help alleviate the unsafe condition that now exists." An ATA spokesman said yesterday there would be no comment on Adams' latest announcement until the formal rule is published.

Adams' ruling can be read as partial repudiation of a position taken by Joan Claybrook, the former Ralph Nader aide who is now administrator of NHTSA. Claybrook consistently ahs defended the antilock rule, pointing to statistics showing that three out of every four persons killed in an accident involving a truck and another vehicle are occupants of the other vehicle. The primary purpose of the antilock rule is to prevent tractor-trailer rigs from jackknifing.

Adams denied he was repudiating Claybrook yesterday and said. "This is one department." Claybrook could not be reached for comment.

Adams also said yesterday that he wanted to support a demonstration program with the trucking industry "that will give us final evidence of the effectiveness of the antilock standard."

That, too, was requested by ATA in its petition. ATA's central complaint recently has been that the antilock brakes are not fail-safe. If the antilock computer malfunctions, the basic preantilock brakes are supposed to operate,normally, but do not, ATA has charged.

ATA also said in its petition that the lack of true fail-safe had the potential to increase the stopping distance from between 158 and 440 feet for a truck traveling at 60 miles an hour. The petition did not say so, but that is 5 miles an hour over the speed limit.

BMCS says the antilock system is new and spare parts are sometimes difficult to obtain in out-of-the-way places. Strict enforcement requires BMCS to stop a truck if the antilock system isn't working.

Adams said he decided to enforce the tractor standard "because tractors receive more maintenance and are at more controllable points. Trailer units, on theother hand, often are left for long periods at uncontrollable points and often in adverse weather conditions.