[Richard]

And the two brake tanks come together kind of at the brake treadle. You may be on to something, it has to be downstream of the brake tank PPVs and common to primary and secondary systems.

If the brake tanks leak down the supply side will try to replenish them and thus be at the same pressure.

I find some air brake schematics that show a double check valve that combines air from either tank and sends one line to the E brake release button. I don't know if we have that setup or not, but it's something to research,