There may be several reasons why your water pressure might drop when you turn on the water. A decrease in water pressure might be caused by a partially closed main valve or broken pipe. Additionally, if the water heater or cold-water line are not properly sealed, additional pressurized air can enter the system and cause a sudden decrease in water pressure. If you think your pressure is low, you should wait 10 minutes or so and check again. If the problem persists, please contact your local utility company.