The screeching brakes have been suppressed on 134 of Metro's 300 subway cars, and silence is promised by April 24 for the rest of the fleet.

Theodore G. Weilgle, Metro's assistant general manager for transit operations, made the promiste in an interview yesterday while asserting that two trains he rode this week stopped silently. Real people have reported similar experiences, especially on the Red Line, which is closer to Metro's primary maintenance yard than the Blue or Orange lines.

The culprit is the brake pad, two of which are required to stop each wheel of a train, a total of 16 per car. New pads have been on order for several weeks and are now being installed as soon as they are delivered. Weigle said in February that the problem would be solved by the middle of March, but slower-than-hoped delivery of new pads invalidated that prediction.

The new pads will cost close to $100,000, Weigle said, and it is just the beginning. Metro does not know how long the new pads will last before they have to be changed again. In addition, Weigle said, Metro is studying a major modification to its braking system both to keep it quiet and to lengthen the life of the components. Federal funds for a $2.3 million trial program are being sought.

"I am confident that major system changes are necessary," Weigle said. ". . . The feeling is that the present brake system is not massive enough. It's safe, but it may not have the longevity designed into it we think is necessary." Weigle declined to estimate the eventual cost of improving the entire brake system, but said it "will be substantial."

Metro's disc brakes are hydraulically actuated. Two pistons drive a plate against an outer brake pad, which squeezes against a disc and an inner brake pad. The vibration resulting from that action is what caused the screech. It has been measured at the same volume as a jackhammer. Metro riders, anticipating pain, regularly plug their ears with their fingers as trains enter stations.

Metro operated for four years without major brake noise problems. The screech began as an occasional wrong note last October, but became a chorus of unmelodic dissonance by the end of November. The reason was that the manufacturers had discontinued making pads containing asbestos, a proven brake-pad component recently discovered to be a cancer-causing agent.

Now, after several months of tests by Metro, the federal Urban Mass Transportation Administration and others, the feeling is that a suitable substitute pad has been found.

During the tests, officials discovered that the disc that the pads squeeze is also a problem, and a much more expensive one to fix. The average brake pad has been lasting about 14,000 miles (or 14 weeks), but can be replaced for less than $25. The average disc lasts about 25,000 miles (25 weeks), but costs $325 to replace. Weigle would like to find something that runs quietly, stops the trains safely and lasts longer.

At the same time, Weigle said, Metro officials are paying particular attention to avoiding the same jproblems in the braking system on new Metro cars now being manufactured in Italy. The first of those cars is scheduled to arrive next year.