Buoyancy tanks leaking

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.