Hey! This is my first post on this forum. I need some help diagnosing why I can't get the rear brakes to bleed or maybe build pressure.

Background:

1. Originally a rear drum brake car, converted over to disc with a bigbrakeupgrade kit to use 2010+ factory Brembos.
2. Willwood 1.125" master cylinder with a small dia. Double diaphragm booster
3. SSBC manual prop valve
4. Factory lines from the master cylinder out.

So far I've done:

1. Bench bleed the master cylinder twice off the car
2. Replaced questionable lines with same size lines.
3. Lashed the pushrod from the booster to master cylinder (was too long)
4. Gravity bled the brakes to start
5. Bled both the rear brakes together with a vacuum pump
6. Bled both calipers separately starting with outside screw, then inside screw.
7. Adjusted the proportion valve and re-bled the brakes
8. Double checked my math and application.

After all of this, I still have little to no pressure coming from the rear calipers. I've moved about a quart of fluid through the calipers in the back. The front calipers work as they should. I can see the pistons move slightly as the pedal is pushed.

Could it be the feed line to the rear is too big? Could it be the rubber hoses to the calipers are ballooning? I don't want to just throw parts at it but I don't know where to look either.

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