A hydraulic braking system that some transit industry officials feel is too trouble-prone is blamed by experts from both Metro and Rohr Industries for causing most of the recent Metro breakdowns.

John Nicita, director of engineering for Rohr's Metro program, and David Gaul, director of equipment design for Metro, agreed in separate interviews plagued Metro operations in July is largely solved.

"That doesn't mean we still don't get sticking doors," Gaul said, "becuase we do, but we think that we have licked the basic problems if we can keep the doors maintained "properly". When doors stick open, the train won't move. That problem was aggravated during rush hours when the doors would bind as the cars deflected downward under full loads of people.

The deflection of the cars was anticipated in Metro's design but the doors were still supposed to open and close smoothly. After some modifications and adjustments, most of them do most of the time.

The brake problem has been more difficult, becuase it has many facets, but if anything goes wrong with the brakes or with the automated safety equipment that monitors the brakes, the train is stopped and locked in position. Although the condition is safe, a stalled train during rush hour s fouls up operations throughout the system. The longer it takes to get moving, the bigger the problem.

First there was the "differential pressure switch," which measured the weight of the load on each truck (set of wheels) on the train. If the difference between the load exceeded certain tolerances, the brakes would lock. Most of the time Metro found, the loads were evenly distributed but the switch was faulty. It has been replaced as rapidly as new parts have become available.

Then there was an adjustment on the device that maintains hydraulic pressure in the braking system. Because of a typographical error in the maintenance manual, a switch was being improperly set by Metro crews, Nicita said. That has been corrected.

Other difficulties have included problems with maintaining proper pressure in a brake control valve, keeping fuses from blowing on a compressor and improperly adjusted bolts on compressor heads.

The most recent major problems the one that causes trains to go into an emergency stop, has been tentatively identified as caused by water in the air line. A filter has been used to solve that. Air pressure is used as a back-up for the hydraulic brake system. If the air pressure drops in the system, the brakes automatically go into emergency and stop the train.

Metro is putting six cars a day through a brake-check program to make adjustments and change parts. "We don't fully understand it all yet," said Nicita, "but I think the incidence of brake trouble will go down."

The only other transit system in the world using hyraulic brakes on rail cars in Bay Area Rapid Transit (BART) in San Francisco. BART is also the only other transit system with Rohr Industries cars.

"We have had difficulty with the brake systems," said BART spokesman Michael G. Healy, "but I would not call our problems monumental."

When the Metropolitan Atlanta Rapid Transit Authority (MARTA) was designing its subway cars, however, it refused to accept an offer from Rohr for a modified Metro car unless the brake system was changed to full pneumatic (air), MARTA officials said.

"We had looked at BART and Metro," MARTA board chairman John Wright said," and our engineers decided to stay with full pneumatic."

"We got into hydraulic brakes," said Metro's Gaul, "because Metro was designed as a fully automated system with rigid (brake) response requirements. Hydraulic systems are more responsive than pneumatic systems."

Metro is restudying its brake specification before it places a second order for subway cars, Gaul said.