WASHINGTON -- The Orioles got the matchup they wanted and the result they dreaded.

The Orioles knew they would be working with a short bullpen Sunday, so they made sure to save southpaw Jamie Walker to face Washington slugger Adam Dunn. That exact matchup came to pass late in the game, and Dunn hit a grand slam -- his second home run of the day -- to give the Nationals an 8-5 win.

That home run, while signaling defeat for the day, also fed into a larger trend. The Orioles are 2-13 in series finales this season, and they're 0-7 when playing the final game of a series on the road. Baltimore (18-26) has had four chances to earn a three-game series sweep but has fallen short each time.

"I don't really have an answer for that," said second baseman Brian Roberts, the second-longest tenured member of the team. "All I can look at is today, [because] I don't really remember the other series, to tell you the truth. Today, we got the lead, gave it up, got the lead [and] gave it up. There's only so many times you can bounce back. ... You've got to come up with the big hit or the big pitch. And we didn't do that."

The Orioles did get a few big hits on Sunday, including a go-ahead home run by Adam Jones in the seventh inning. That two-run shot came on the heels of a Washington rally and gave the Orioles a short-lived 5-4 lead. Chris Ray (0-1) came out to protect that advantage and fell into trouble by allowing two quick singles.

Washington moved the runners over with a sacrifice bunt, and Baltimore elected to walk Ryan Zimmerman and load the bases. That brought on the long-awaited matchup between lefty slugger and lefty reliever, and after getting ahead, Walker grooved a fastball that Dunn sent to the back wall in the Baltimore bullpen.

From there, the Orioles went quietly and ended their road trip with a 4-6 record.

"We didn't have very many options with the bullpen," said manager Dave Trembley. "I thought we had one opportunity in the matchup, and that was it. He made some good pitches. I thought we might have had him, and he just fouled off a couple. Then he got one and didn't miss it. I have to go with what I have today, and that's what I did."

"I just made the one bad pitch. Actually, I made two," added Walker, talking his way through the key at-bat. "That hanging slider that he fouled back was probably the pitch I thought he'd hit out. We were trying to go in right there, 2-2 count, and my ball just cut and ran right back over the outer third, right in his homer-zone. He did what he's supposed to do. I made one bad pitch, and it cost us the game. That's the life of a reliever."

And in this case, it's the life of a reliever without a role. Lefties are hitting .400 (8-for-20) off Walker, and the veteran has allowed two home runs -- one to Dunn and one to Robinson Cano -- in the early going. Now, if Walker can't begin to retire left-handed hitters, he may tempt Trembley into trying something different.

"That's what we expect -- for him to be able to match up in situations, left against left," said Trembley of Walker's role. "It hasn't worked of late. I guess we just need to do a better job all around."

Baltimore had led for most of the game, using a two-run triple from Nick Markakis in the third inning to push ahead. The Orioles scored again in the fifth, but Dunn put the Nats ahead with a two-run homer in the sixth. Baltimore answered right back on the Jones homer but couldn't make it stand up for the win.

The Orioles also had chances to sweep their first three series of the season, only to fall in the finale. Baltimore took two of three from New York, Tampa Bay and Texas in the first two weeks of the season but lost all three finales by a one-sided margin. In all, Baltimore has been outscored 49-16 when going for a series sweep.

"I don't know how to answer that question or put a finger on it," Trembley said of the larger trend. "We've got a 3-0 lead. Then the other team goes ahead and then you come back late in the game and you're ahead, 5-4. It could be the first game of the series. It could be the second game of the series. These things seem to happen the last game of the series, and I don't know why. I really don't have an answer for it."

At any rate, that late result obscured an erratic start from Brad Bergesen. The rookie pitched four scoreless innings before giving up back-to-back doubles in the fifth. Washington starter Shairon Martis drove in a run with a single, and Bergesen went on to allow Dunn's first homer of the game in the sixth.

"I go out there and give it 100 percent out there," said Bergesen. "Looking at film, I see that in some of those later innings my arm has dropped down a couple times, and I leave that pitch over the plate. I feel like every outing I'm getting deep in the game and I'm a pitch or two away from getting out of it."