The fact you get the same amount of water in regardless of how long your sub hasbeen in the water suggests it is actually sucking water through a seal or fitting somewhere. Each time you take the sub apart and dry it out, the dive unit goes back together and the whole thing is at ambient temperature. As soon as you plunge it into cold water, the air inside the wtc cools down, contracts and causes a partial vacuum. This drop in pressure causes water to be drawn in through your defective seal/ fitting and since the air contacts by a more or less fixed amount, the same mount of water will be drawn into the unit each time, regardless of duration of immersion.
Sounds like a good contender for a pressure nipple so you can apply a little positive pressure to the wtc, this will not only show up your leak but will prevent water ingress.
Chris