Just bought a Raleigh Bob with flat tyres. Inflated with CO2, but a week later, they were almost flat again. So, bought new tubes and swapped over - checked the old tubes with a little pressure from a hand pump and a full sink but couldn't find a leak. When I filled the new tubes I put some hand pumped air in first because a 16oz cartridge didn't quite get a 26"x2" hard enough, finished off with a cartridge per wheel and two days later they're noticeably softer again. I did read that there's more leakage through a butyl inner tube wall with CO2 but is that REALLY enough to noticeably soften a tyre in 2 days?