Why are my Aprilia Mille's tyres deflating?

Q. I have bought a 2002 Aprilia Mille that is having tyre problems – they’re deflating for no reason. I have put new tyres and valves on and the problem is still there. Motorcycle tyres for sale. The front tyre was going down but seems OK since I changed the valve.

The rear now seems to be the problem, it can go down at any time, even when the bike is on the paddock stand for a few days, It’s very frustrating.
Johnp4455, MCN forums

A. If you hadn’t changed the tyre we’d say it was a slow puncture or two. But with new rubber and valves, the finger is pointing at the wheels and the tyres not seating on the rim properly.

Fill a bowl of water, drop the wheel off when it is inflated and rotate thewheel through the water to see where any air is coming out. If it isn’t sealing, mark the leak position and take it back to the tyre fitters to getthem to clean up any corrosion and maybe use a different lube to pop it on.