I'd also be interseted in what happens to the weaker vent when you introduce pressure/restriction to the
outlet, whether it stays balanced or starts to do what you just mentioned. How would you test for it though, fit some blocks and rads inline and see what comes out the end?, if it's still got good pressure all's OK...
If it buggers up there's always the ratchet method, did you try it?, did it work and flip the right way?, or did it keep stalling the wrong way?...
PS, a valve/flap at the Y would work as well, when alls well it would be 'central, if the weak flow gets to low the strong one will force the valve shut. you'd have to make the Y & valve yourself but I'm sure it's 'doable'
. worth it too if that's what it takes...
PPS, you could just use 'one way' valves in the two tubes that lead to the Y fitting
, I don't know how restrictive they are though?. I'm sure we could make a 'passive' one that works of the water pressure according to flow direction, rather than a spring type one. would have near on zero restriction if made oversized bore wize...