On the 130 models, you should be able to poke a small hose all the way down the leg to get the oil out, past the diaphragm level. It may take some hunting around. On the 120 models that's not possible, as you'd have to guide the hose through two 90 degree bends, which is very unlikely to succeed.
It's possible for the gasket between upper and lower leg allowing a leak between the seawater and the oil channels inside the leg, but that's fairly unlikely if you've followed the instructions to the letter and didn't damage the gasket during assembly. Much more likely you'll indeed find a problem with the shaft seals, either damage from fishing line, strands of rope or wear on shaft or seals.
It's also possible the leg has corrosion that opens it to the sea, but that shouldn't happen if there's still any anode left. In that case, the oil would be bubbling out of the open filler cap, pushed by the water pressure, so that's likely not the case here either.
Unfortunately only a haulout will tell you what's really going on.
