Very common, I've seen at least 10-or 15 just like that.Those almost look like freeze fractures... filled with water and froze...
My guess is stress fractures, vibration and heat cycling / expansion contraction of the air inside.
Why I ALWAYS hot water dip floats on overhauls. Maybe it's an up Nort thing?
It's very thin shhet brass subjected to a deep draw forming process.
https://www.thomasnet.com/articles/custom-manufacturing-fabricating/deep-drawing-brass/
Disadvantages Of Deep Drawing Brass;
Stress cracking: Brass is more susceptible to stress corrosion cracking (also called season cracking) than other metals.
One might conjecture that a float made of two equal halves, soldered in the middle might have been better than a cup and cap design, reducing the draw depth by nearly 50%. Also would place the heavy solder joint in line with the support bracket instead of cantilevered reducing stress loads.
There will be as many as 7 or 8 cracks all the way around the float.
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