I have recently repainted an old Mirror my dad built. I gave the buoyancy tanks a test with a hosepipe to see if they hold pressure. The pressure eased quite quickly and there was a hushed whistling / hissing. Listening round the outside of the boat I couldn't hear where the air was getting away, nor inside the boat.
I wondered if it was blowing inside the boat between the aft buoyancy tank and the side tanks. Is this common? Do the tanks have to be sealed from each other to pass a buoyancy test? If so, how do you go about fixing this on an old wooden boat?
This is a common problem with older boats built with old style glues. In fact my first boat suffered with it from the day I built her. It is a requirement of measurement certificate I believe but not realy detrimental if not racing at a high level. A cure can be tried if you have access to the tanks by way running liquid epoxy resin into the leaking area but I think it is more by luck than judgement that a permanent cure is achieved this way. If it is an issue for the boat to fully meet measurement rules then the only permanent fixture is to lift the tank tops and reseal all internal joints with epoxy and cloth tape before refitting the lids with a thickened epoxy resin glue.
I agree with Bob, older boats often leak between the aft tank and the side tanks. The aft bulkhead panel has oversize cutouts in it to allow for the stringers which runs along the topside panels and support the outer edges of the decks. If the gaps between the stringers and the bulkhead are not sealed when the boat is built, this problem will occur. You can check this is what is happening by putting the bungs & hatch covers on the other two tanks and treating the three tanks as one. You will need to blow in more air (maybe three good blows) and leave time between blows for the pressure in each tank to equalise (if you can hear whistling that suggests the hole is quite small).
If this fails, air must be escaping somewhere. First place I would look is at the stowage bulkhead panel (the one with the cuddie cutouts). This has the same oversize cutouts for the stringers as the aft bulkhead panel, but at least you can get at the gaps via the cuddies (not easy, but if you put the boat upside down resting on trestles etc. and work underneath I find it easier. Thickened epoxy in the corner where the stringer emerges should work. If you put your hosepipe in the side tank drainage hole and suck gently, you will draw the epoxy into the gap which will help seal it up.
If you want to cure a tank to tank leak without taking the decks off, you could put a hatch into the back tank, or hatches into the two side tanks (towards the aft end) and use these to get at the gaps, but you will only be able to get your arm in, so won't be able to see what you are doing.
By the way, class rule 4.1 allows the aft and side tanks to be treated as one tank for the purposes of a buoyancy test (a fairly recent change), so tank to tank leaking need not stop you getting your boat through the pressure test.
Checked again today and the tanks do hold air if I put bungs in the side tanks. Just need to put a bit of epoxy in four screw holes in the foredeck where I have removed the bracket from an old Sidey rigid pole spinnaker uphaul/downhaul.