I had a similar problem on a Wayfarer World.
You need to pressurise your bouyancy tanks.
This is how I did it.
You need a standard dinghy
drain fitting and plug
, a domestic vacuum with an outlet nozzle, a length of plastic pipe that is a push fit into the drain fitting and some duct tape or similar.
Fit the drain fitting into the tank that leaks
somewhere that it won't get in the way.
Push the plastic pipe into the fitting, connect the other end of the pipe to the exhaust
of the vacuum. On my vacuum the hose connected to the exhaust
so I just pushed the plastic pipe up into the vacuum hose.
You don't need much pressure.
Start the vacuum, I found all my leak points just by listening to the escaping air, but soapy water would make sure.
Then all you have to do is fix the leaks ;-)