A ship was at anchor and the crew were training with a free-fall lifeboat. After the life boat had been launched and the engine started up, there was a negative pressure and the crew of the boat could not open any of the covers. The engine was stopped immediately and after about 20 seconds the pressure had evened out sufficiently for the covers to be opened. It turned out that both pressure-relief valves were corroded and had stuck closed. The harsh environment in combination with poor valve design and lack of main-tenance was deemed to be the cause of the incident.