line lock orientation

10 x 2s will not resist power very well, through the proportioning valve. The rear pressure is limited to about 15/20 percent of the front, 25% tops. At 400psi on the front, the rear will get about 80 psi, and thru 7/8 wheel cylinders, you would get 48 pounds of force delivered to the shoes. That is not much at all.
I weigh 220, and each sneaker gets 110 pounds of force,when I'm standing still; about 2.3 times as much. And a pair of my sneakers has about 30% more frictional area than all four rear shoes.And that is if the rear shoes fully conform to the drum, which is highly unlikely.
Jus saying
Oh chit. you want to keep fluid out.
In that case, might I suggest a ball valve?
Oh jeepers that won't work either.
See if you are able to prevent the fluid from moving out of the M/C, and into the rear line, then the Brake pedal will only move far enough to close the compensating port, and then you have a hydraulic lock situation. The piston cannot continue to move as the inter-piston area is full of fluid and cannot move, nor can the forwardmost piston due to the closed valve. Therefore nothing moves, no brakes at all. And no brakelites probably either.