The air hose hisses like a cobra, like a fuse, slowly pressurizing the sleeping city. There’s a gray GMC truck behind the empty garage, parked on buckled concrete in the alley, past the racks of car tires and hubcaps. The truck is rocking on its axles. Something’s inside and wants to get out.

The tire mechanic unlatches the door and out tumbles a pit bull named Bo, who darts toward the darkness until his chain-link leash catches. There’s a whole world of alley vermin to chase, but Bo settles for a seat next to his master, whom he loves.

“Some people get these animals as a status symbol, but I don’t care,” says Dennis Parker, kneading Bo’s sinewy chest. “I didn’t care he was a pit. I just want him to be a dog.”

Man and beast sit, sentinel-still, in the alley under a fog of amber floodlight. They view the night through the garage portal of Mac’s Tire Service, which promotes itself as the only 24-hour tire service station in the District, Maryland or Virginia.

It’s a little after midnight. Any kind of character could turn in to the garage at any time from Florida Avenue NE. A surly cabbie. Drunk clubgoers who’ve destroyed their vehicles or whose tires have been gutted by a serial slasher. Down-on-their-luck motorists who can’t afford a replacement tire, who are towed in from Richmond or Annapolis. A combative boyfriend who’s too embarrassed to admit in front of his girl that he can’t change a tire.

“Seems like some of the guys don’t know how to do anything manly anymore,” Dennis says.

He was raised by his grandparents on Martin Luther King Jr. Avenue. He thought about joining the military but started trucking school because he felt pulled to the open road. When he fathered a child at 16, he says, plans changed.

Ambulances coast down Florida Avenue, red lights twirling, sirens off.

The breeze carries the scent of rubber.

A boombox slow-jams 102.3 FM.

The garage walls sweat.

“Sometimes life can get in the way,” says Dennis, 45, squeezing a plastic bottle of Rock Creek Fruit Punch. “You can have a dream about going up and down the highway, or flying out to some foreign war, but sometimes you gotta have $11 for Pampers.”

He dropped out of trucking school. Had run-ins with the law. Had six more children and became a mechanic to support them. Learned to control his temper, to fight his battles on the inside. In 2000, he started at Mac’s, a 30-year-old, family-owned business. Last year, he bought a house down in Charles County, a bucolic retreat for a man raised in the city.

“I figure if I have to work around characters, I don’t have to live around characters,” Dennis says.

While tightening ball joints, he fantasizes. Wouldn’t it be great if he got paid by GMC to test-drive new models? If he won the lottery, he’d take trucks out to the Grand Canyon. He’d take trucks to Alaska. He’d see America from a lofty perch, at 80 mph, pulling 80,000 pounds up steep hills without slowing, a herd of horsepower at his feet.

There’s a whole world of highway to cover.

Dennis fixes wheels in the dead of night so others can cover it.

Last Friday, he got fitted for a tuxedo for his baby girl’s October wedding.

The air hose hisses.

About 2:30 a.m. a green minivan lurches into the garage. Hairdresser Rebecca Fowlkes and her family left St. Albans, W.Va., at 2 p.m. that day to sight-see in the capital. A front tire blew on the way out to their hotel in Laurel.

Dennis stands up from his post in the alley, greets them, quietly changes the tire in a matter of minutes for $115.

“I tell you what,” Rebecca tells him as her family climbs back into the van, “I wouldn’t know what I’d do if not for you.”

Dennis nods. He waves as the minivan trundles off through the alley, headed for the open road.