Performed a heating pm on an oil boiler. Brushed and vacuumed. Replaced the oil filter, nozzle and strainer. Started and checked the safeties. Performed a combustion test. 6.8% O2, 5 ppm, 0 smoke, 82.4% efficient System is operating properly. While performing the maintenance, noticed (2) hi-vents were leaking. Recommend replacing them. Drained the pressure from the boiler and replaced the 2- hi-vents at the boiler. Filled and purged.