Like most of his fellow travelers on the Colonial Transit evening bus from downtown Washington to Prince William County, Cecil Sears has become blase about windows that don't open, rush-hour breakdowns and mysterious groans from faltering transmissions. Still, the day a rear bus wheel fell off, even Sears was goggle-eyed.

"I kept hearing this grinding sound, really loud, from the back, and then I heard this noise," he remembers. "The wheel came off, and went bouncing off down Dale Boulevard. Cars were swerving to avoid it. We all got off the bus, no one was hurt, and actually it was kind of nice -- a lot of people riding by in cars stopped to pick us up. I remember I got a ride home in the back of a pickup truck."

Every commuter line has horror stories, but lately on Colonial--a financially troubled line struggling to stay afloat--a day without anxiety is like a day without sunshine. Because Prince William County, the fastest growing county in the nation, has steadfastly shunned high-subsidy Metrobus service, those of its 10,711 daily downtown commuters who travel by bus would be stranded if Colonial didn't exist. But those who say they can't live without Colonial are beginning to wonder if they can live with it.

"It's gotten to the point now where you get on the bus in the morning and think, 'Okay, do I want to sit in the front, and take the risk of flying glass, should I sit in the middle, where it's hard to get out, or in the back? No, that's near the engine," says Colonial commuter Becky Holt.

A few weeks ago a bus with a wobbling tire was waved off I-95 by an attentive truck driver. It was discovered that only two lug nuts remained on the back wheel, and the tire, riders say, was tilted at a 45-degree angle. Riders boarded the next Colonial bus that came along, only to have that one collide with a Volvo. The engine of another bus recently caught fire.

Company president Hal James acknowledges the mishaps but reminds that Colonial has never had a fatal accident in 37 years of operation. He blames Colonial's problems on the impossibility of operating a modern-day commuter line without federal or state public transit subsidy.

Commuter bus lines have long been considered marginally profitable and recent government efforts encouraging such ride-sharing innovations as subsidized van pools have only added to James' woes.

James and his wife bought controlling stock in Colonial in 1979. Once critical of the line's old management for not turning a profit, he now finds himself in the same predicament.

"We have a chronic shortage of buses," he says. "We simply have not been able to make enough profit to buy new buses. Our dispatchers do a constant juggling act."

That juggling act includes 56 buses covering 26 runs each workday, ferrying passengers back and forth along Interstate 95 and Rte. 1 to the Pentagon, Crystal City and downtown Washington.

On a good morning, the 25-mile trip can be made in just under an hour. But the good mornings are becoming fewer.

Breakdowns are common. Buses run out of fuel during rush hour, interior lights don't work, roofs leak, buses are often late. "It's gotten worse," says passenger Lee Slayton.

"Just as long as you remember to keep your umbrella up inside the bus when its raining," says Judy Donovan, a three-year Colonial veteran.

The company's daily commuter ridership has dropped from about 2,000 in 1978 to under 1,000 last year and the company has canceled some of its runs.

Colonial charges $23 a week for round-trip passage to Washington, money that must pay for everything. Ticket sales for transit authorities such as Metro make up only about half the system's budget, with government subsidies paying the rest. James says passengers spoiled by subsidized mass transit complain when fares are raised.

So the company barely operates in the black. Most of its buses are 20 to 30 years old. Even with 14 mechanics working two shifts in a garage that's open all day and night, the buses break down. And those in best repair tend to get diverted to charters, which James says accounts for 60 percent of the company's business and nearly all of its profit.

The federal Bureau of Motor Carrier Safety cited Colonial four years ago for operating unsafe equipment and again in 1981 for inadequate maintenance recordkeeping. The bureau's 149 inspectors, watchdogs for 4 million transit companies nationwide, haven't been back to Colonial since 1981, but federal inspector Merrit Sargent says inspectors will be back soon because the bureau has been getting safety complaints.

Under Virginia law buses must be inspected once a year at state-approved bus garages. In addition, federal regulations require that companies inspect their own buses every 90 days and keep written reports. James says his drivers also inspect the buses each morning.

This year, Prince William County received a $1.35 million grant from the state of Virginia to buy 20 new commuter buses and lease them at a cheap rate to a private operator. The county has not yet decided who will receive the bus lease, but James--and some of his customers--think Colonial deserves the buses for sticking by county commuters.

"They provide flexibility," says Sears. "Even if the first bus doesn't come, you can always catch the 'desperation run' at the end of the day. Colonial does all right with the equipment they've got. Suppose I had to do all this driving myself. There'd be no way, I'd be a wreck."