Service on Montgomery County's Ride-On bus system, a five-year-old government-operated minibus operation that carries about 14,000 passengers a day, was cut back by 15 percent yesterday because of persistent maintenance problems.

County officials said a combination of "severe mechanical problems and a shortage of buses" forced the curtailment in service, which affects about one-third of the percent 22 routes.

Under the new schedule, two midday routes have been canceled and six others somewhat curtailed, and rush-hour service to and from the Silver Spring Metro station has been cut back. Service also been canceled on five routes Saturdays and four routes Sundays.

At the root of the problem, transportation officials said, is the unexpectedly high demand for the bus service, a demand that has caused more wear and tear on the 35-seat minibuses than they were designed to withstand.

Since the opening of the Silver Spring Metor stop in February 1978, ridership has escalated dramatically, as have maintenance problems on the system's 69 buses. In the year ending in June 1977, the buses carried 967,000 passengers; by June 1978 the annual ridership had jumped to 2 million, and transportation officials estimate that 4 million people will use the saystem this year.

"I personally think the main problem is that we're running the equivlent of a large metropolitan transit service with a light-duty piece of equipment in heavy-duty service," said county transportation planner Ed Daniel.

As a result, drivers of the buses have charged, they are being forced to drive unsafe vehicles that are plagued by braking and steering difficulties. Recently the system has averaged 10 accidents a month, largely "fenderbenders," according to the county's safety inspector, William Flannery.

Flannery said yesterday that "90 percent" of the drivers' complaints are exaggerated. Nevertheless, he added, he would rate the overall safety of the system no better than "7.8 on a scale of 10."

But, Flannery added yesterday, "If I really had a safety problem, I'd stop the system.... I woundln't let the buses go out."

The concerns about the Ride-On System came to light yesterday as transportation officials held an emergency meeting with Robert Wilson, the county's chief administrative officer, to discuss the maintenance problems and equipment shortages.

Increasing mechanical failures and a shortage of mechanics have resulted in an overload for the county-operated garage that services every countyowned vehicle, from police cruisers to snowplows, according to transportation officials.

"The demand is greater than our capacity," said assistant maintenance chief Bob Peers.

Because of the overload, "buses are not maintained totally properly," he said. Another transportation official said the repairs are rushed and incomplete, to keep 48 of the 69 buses in service at all times.

These problems have been compounded by premature wear and tear by some drivers who brake and accelerate too abruptly, and by an imperfectly designed vehicle that combines a doby made by one manufacturer and an engine by another. In addition, there is the constant stress of winding along curving local roads and up and down steep hills an average of 14 hours a day, transportation officials said.

"What we are using is what was available at the time in the way of these buses," said Peers. "The demand for this type of service wasn't there for the industry to take notice of it. But now the demand is greater and we're getting bettern buses."

As a short-term solution to the maintenance backup, the county transportation department put the service cutbacks into effect yesterday for an indefinite period. Transportation officials said they expect some improvement in service with the arrival of 32 new heavy-duty buses that should go into service in early summer.

Meanwhile, county administrator Wilson told acting transportation chief Anthony Kanz yesterday to make additional recommendations today, exploring the possibilities of leasing or purchasing new equipment under emergency procedures and of expediting repairs.

Wilson said it is possible that County Executive Charles W. Gilchirst may recommend a supplemental $2 million appropriation in next year's budget for a new maintenance garage that has been planned for several years but never funded.

"This system wasn't anticipated to be so popular," Wilson said. "Now we're going to have to accommodate it to the demand."

Gilchrist's 1980 budget request also asks for a $1.9 million in additional funds to expand the Silver Spring and Gaithersburg Ride-On services to an anticipated 1.1 million new passengers.

The Ride-On system was started in 1974 to augment Metro routes with minibuses that could travel deep into neighborhoods on narrow roads where the larger Metrobuses could not maneuver, and at the same time, provide frequent midday and weekend bus service.

The Ride-On program is totally funded by the county -- at a cost of $3.6 million this year -- and passengers pay 25 cents a ride and can transfer to and from Metrobuses for a small additional charge.

These routes are affected by the cutbacks: 1, 4, 7, 10, 11 and 14 during rush hours, and various trips on Routes 3, 16, 17, 18 and 19. Routes 21 and 22 are canceled on weekdays. Saturday service is canceled for Routes 6, 10, 14, 21 and 22 and some cutbacks will occur on Routes 16 and 18 on Saturdays. Sunday service is canceled for Routes 1, 10, 14 and 16 and revised Sunday schedules will go into effect for Routes 9, 12 and 15.

For specific information, call the Transit Information Center at 468-4100.