By holding their opponents to less than 100 points for the fifth straight game, the defensive-conscious Washington Bullets rallied to beat the Chicago Bulls, 104-99, last night for their third victory in their last four games.

Utilizing a half-court, trapping defense, backed up by the shot-blocking of Rick Mahorn, the Bullets forced Chicago into 25 turnovers, which they converted into 30 points to the delight of 6,715 at Capital Centre.

Once again, it was Mahorn and Jeff Ruland who carried the Bullets down the stretch. Mahorn led the scoring with 17 points, but it was Ruland, coming off the bench, who provided the impetus for the victory.

Ruland scored nine of his 16 points in the last 10 minutes and led the Bullet rebounders with 10, including four off the offensive boards.

"I'm feeling better and better each game," the rookie from Iona said. "Ricky and I played together a lot this summer and we feel comfortable playing with each other.

"Usually, when we're in there, we cause a mismatch for somebody and we just try to take advantage of it. The way we play, our positions are interchangeable. We can both post up, depending on the defense."

Although Mahorn and Ruland supplied the one-two punch at the finish, they never would have had the opportunity if a pair of former Los Angeles Lakers hadn't carried the burden earlier.

Jim Chones, who wasn't even used in two of the first six games, came off the bench when Mahorn picked up his second foul after only four minutes and scored eight points in the first quarter, which ended with Ricky Sobers tossing in a desperation 40-footer for three points to put the Bulls in front, 26-24.

Chones came to the rescue again in the second quarter when Mahorn was tagged with his third foul trying to contain Artis Gilmore, who finished with 24 points. Chones ran his successful string to six straight shots and scored eight more points to give him 16 by intermission.

"It wasn't hard coming off like that," said Chones, who had played a mere 69 minutes this season. "I know what I can do. There's no pressure on me."

Despite Chones' scoring spree, the Bullets trailed, 49-48, at halftime because Gilmore sank a desperation three-pointer when he caught the ball near midcourt, took one step, turned and pushed the shot at the basket.

"We were playing great defense and then still had to overcome those two three-pointers," said Coach Gene Shue, who could afford to laugh about them later. "Those things have a way of taking a little out of you."

It was Brad Holland, the other half of the Los Angeles connection, who took over the scoring load in the third quarter when the Bullets outscored the visitors, 29-24, to take the lead for good.

The three-year veteran, who has been starting in place of the injured Kevin Grevey (pulled thigh muscle), made four shots in a row and had 10 points for the period.

"It was bound to happen," Holland said after breaking out of his shooting slump (11 of 34). "It was encouraging that the coach ran some plays for me. That helped my confidence and, of course, the longer I stayed in, the better I felt."

The Bullets never trailed after Mahorn broke a 67-67 tie with a 10-footer with 2:29 left in the third quarter. Holland followed with two free throws and the Bulls never reduced the four-point deficit.

With Ruland scoring four times, Mahorn making a jumper and Frank Johnson driving for a layup, the Bullets increased their advantage to 94-83 with five minutes to play. Then they grew tentative and almost blew their lead.

After Don Collins' 10-footer put Washington in front, 100-86, the Bulls scored the next eight points before John Lucas sank two foul shots. Reggie Theus made a three-pointer to cut the lead to 102-97. The defense tightened again and the team wrapped up its second straight victory.

The Bulls are 0-5 on the road this season . . . Theus made six straight shots in the first quarter and was the game leader with 27 . . . The Bullets will be idle until Tuesday night when Milwaukee will play here.