The #10lbs keeps the cylinders charged so the shoes stay closer to the drums for the next stop. Without it the springs will push the pistons into the cylinder and thus have to travel out more before they touch during the next stop.
Nope. Complete BS and yet another nonsensical thing that circulates throughout the web.
A 10# valve is NOT going to overcome drum brake spring pressure and keep shoes closer to the drum friction surface.
No how, no way.
The residual valve merely keeps pressure in the line so that with the next brake pedal application, you don't have to "recharge"
that brake line with fluid.... that may have drained back into the master cylinder.
This is important on most drum applications and really important when the master cylinder is below the level of the drums,
like on some classic hot rod applications.
ex-Global West GM
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