When you have them take the tubes out make sure where the bead of the tire seats on the rim is clean.They also need to be smooth free of any nicks and if clean and you will not have a problem with them losing air.

PS if you can feel a nick or scratch on the inside of wheel lip where the tire seats it needs to be sanded/filed smooth or some times a coat of $.99 Wal-Mart spray paint will make it smooth.When you hear someone complain about their tires losing air 99.99% of the time that is the problem.