Leaking float bowls (CB400F)

[Yetanotherbike]

I'm sure that some people on here may have encountered this before, but thought i would share it.
On my recent purchase, one of my tasks was to sort the leaking float bowls. Having done this loads of times, I was filled with confidence,(stoopidly). Cleaned the float needles and seat, checked the floats were actually floating, set the float height and they all overflowed.
Replaced the float needles and seats, checked the float height again and to my annoyance they all overflowed 
I was starting to get desperate now as it meant that I would have to now engage my brain. I decided to lower the float height as an experiment and it achieved the same result. I made an adaptor to attached a tube to the float bowl so I could observe the actual fluid level. To my surprise, it was well below the top of the overflow. On a whim, I removed the bowl and filled it with petrol and it leaked through the overflow! This was a surprise to say the least. I checked the other three bowls and they all did the same. I checked the brass tubes with a magnifying glass and realised that they all had hair line cracks in them.
So my question is, how did it happen? I wondered if it was my ultrasonic cleaner, resonance from the engine or water in the fuel that had frozen at some point. I was thinking over drilling it out and soldering in some 3mm brass tube or just soldering the crack with solder and drilling it out again. Any thoughts good people.

[K2-K6]

Yes occasionally you get one, but strange all 4. Unsure of original cause.

Don't think you can solder to the float bowl material (zinc based) as original looked pushed in.

Possibly solder the tube to seal and clean bore to clear any blockage after.

If there's room in there,  sleeving with silicone tubing (remember the old style bicycle valve had a little rubber tube over their stem covering two holes) would probably be the easiest way to give reliable competent fix.

[Oddjob]

I seem to recall there was a member who replaced the overflow tubes entirely with new brass, I seem to think they were actually 2 pieces but could be wrong.

Maybe search and see if you can find it.

[Laverdaroo]

I replaced mine on my RD350 years ago and I just saw Allan Millyard do it to a set of S1 carbs hes building up on his you tube channel.

If you heat the boal around the base of the brass tube where it inserts into the bowl being careful not to melt the monkey metal, as Bryan calls it , an then with a pair of mole grips , grab it and twist. It should come straight out.

To replace, heat the area around the hole line the brass tube up in the hole and tap with a hammer a couple of times until its seated and there you go, they're in and once cool, seated and leak tight.