Hello and welcome to our community! Is this your first visit?
Update: I have replaced the MAV thread bits already, they are tight (magic 13mm hex key), I've had it in the bath for bubble tests, the water is in the exhale counterlung not the canister and it is watery, not gloopy so I don't think it is just me drooling too much! :-)
Going to have another look at the mouthpiece. I've got a regular one and a BOV, I'll swap 'em over and see if the problem repeats or not and see if I am being 'loose lipped' as it were :-)
Are you consciously moving water into the lung? If your not draining the loop back into the lung via the water trap it must be a lung fitting letting by , if you are moving water back , I usually move fluid back when it starts “banging” against the flappper valve then check out the mouthpiece.
New TDF Member
Just the problem I had. The unit would hold negative and positive checks but got water in the bottom of the canister with a good bubbling sound on breathing. Luckily the unit still breathed well enough to get back to the boat without bailing out.
I eventually tracked it down to the counterlung overpressure valve. I had some contamination under the internal silicon mushroom valve that normally prevents anything entering the counterlung. Outside the mushroom valve is the rubber disc and spring that vents over-pressure in the lungs. With the overpressure valve screwed down to its test position, the rubber disk sealed enough to hold negative. When the overpressure valve was released to its normal dive position, the spring pressure wasn't enough to seal against negative pressures ( which its not designed to do ) and I got water ingress. I eventually caught it by putting a negative pressure in the loop, then pulling the dumps. When I pulled on one side, it held negative. Pulling on the other side, I got a leak. I changed to a spare overpressure valve - quite a tricky job and the problem was solved. A temporary fix was to dive with the leaky valve screwed down, relying on the second valve to prevent excess pressure in my lungs
I had it in Florida, almost half a liter in the exhale lung AND cannisterbottom after each dive. But it holded all tests. Back at home where it was colder: no issues anymore. The reason: the mouthpiece bite.
Another leak I had was that the plastic part of the hose for the mav on the bmcl was not tightened enough anymore. But here the unit did not hold pos-test, but neg-test was good. No water inlet during a dive.
I had a tiny cut in a counterlung.
Under a vacuum the cut was pulled against the rubber of the MAV fitting and did not leak, but it let some water in in normal use.
Inspect the lungs carefully.
Thanks for the suggestions all, will hopefully have chance to test/fix this weekend.