United Airlines, Pressurization Issue

A United Boeing 767-300, registration N653UA performing flight UA-854 from Houston Intercontinental,TX (USA) to Lima (Peru), was enroute at FL350 about 180nm southeast of Houston about 40 minutes into the flight when the crew decided to return to Houston due to a problem with one of the pressurization valves. The aircraft descended normally to FL300, then FL220 and landed safely back in Houston about 170 minutes after departure. A passenger reported the captain announced they were returning to Houston because of a problem with a pressurization valve. The aircraft entered a hold to get down to Maximum Landing Weight and landed safely. A replacement Boeing 767-300 registration N649UA reached Lima with a delay of 7 hours. The occurrence aircraft remained on the ground for 24 hours then resumed service.

Source: http://www.avherald.com/h?article=4ac66da2&opt=0