Heavy rain in the Sierra foothills pushed a small dam within San Francisco’s Hetch Hetchy water system to the brink of failure Thursday, sending a brief scare through the rural region where roads were closed and a few dozen residents were forced to evacuate. Officials at the San Francisco Public Utilities Commission said the danger in the area, west of Yosemite National Park, had diminished by nightfall as the storm gave way. There was no interruption to water service for the agency’s 2.7 million Bay Area customers.