A promise from Montreal's transit corporation to improve service by adding more buses fell through this week because of a shortage of vehicles.

The union representing Montreal bus drivers says many of its members were paid to work shifts this week but ended up sitting around because there weren't enough buses to drive during Monday and Tuesday rush hours.

The Montreal Transit Corp. added 145 additional departures per week on its green, orange and blue metro lines starting Monday. It also extended morning and afternoon rush hour schedules for several busy bus routes. The extra service will cost about $20 million a year.

But the transit corporation admitted it is running the system with bus shortages. "I can tell you that [on Monday], we were missing between 100 and 120 buses on the network," said MTC spokeswoman Isabelle Tremblay.

"This morning we were missing 105 buses. Now unfortunately, this situation has been going on for a few months," she said.

Part of the problem is a fleet of older buses that keep ending up in the shop, Tremblay said. "Between 1995 and 2000, we bought about 450 of the first generation of these low-floor buses, which continue to be a headache since they are three times more likely to break down."

The situation has been exacerbated by an ongoing labour conflict with maintenance workers, she said. "There are a few factors that explain the lack … of buses at the peak of rush hour," she said.

The union representing transit workers said the problem can't be blamed on maintenance workers. "It's not the first time they have big, big problems," said union spokesman Sylvain Pilon.

"The cold reality is that [Monday] morning we only had 200 buses put on roads, for a total of 146 hours [of service]," which falls short of rush hour needs, Pilon said.

The only solution is for the MTC to buy new buses, said bus driver union spokesman Tom Mouhteros. "It's not working at all. They need to put more buses on the road. Find and buy more buses."

"The sooner, the better," he said.

The MTC says it plans to buy about 100 new buses this year and is talking to Nova Bus, which built the older vehicles, to find a way to keep them on the road.

The extra service was introduced as part of the MTC's efforts to increase ridership by eight per cent in the next five years, according to the province's green plan policy.