New ballast system
This system uses a flex bag and a automotive 12 volts pump.
The difference with other systems is that it almost doesn't compress air into a tight compartment, instead the air inside escapes through a small tube, glued to the bag. When emptying the ballast, water is pumped out; as the tube is (now) submerged, the water tries to enter the bag but it can't because the tube has a very narrow inner diammeter (1mm), so a vaccum is created inside the bag and it shrinks, making the submarine surfaces again once some amount of water is pumped out.
Once the small tube gets into contact with the air again, if the pump is still running it sucks air into the bag, emptying it more quickly, and the sub is on the surface again.
A clunk is needed to be sure that water will be pumped out from the bottom of the bag, and the tube has to be raised enough to let the bubble escape through it.
Simple, reliable, cheap, no valves needed, just a flex bag (I use a serum bag, those you see at the hospital), the pump has to be a geared one but no need for a strong one, since it doesn't have to cope with the pressure created when you pump water compressing air in a pressurized bottle; the pump is designed to run on 12 volts, I make it run in 6volts with success!
I'm gonna post a vid as soon as possible.
Regards
Robert