I have done everything by the book.

1. 1 & 1/8 bore master cylinder.
2. Brake pedal/pin mod (moves pin closer to pivot point).
3. Dual Rear Caliper Discs (for boost on foot brake).
4. All 3/16" hard line, no flex lines for better pedal feel.
5. 2lb residual valve on the rears as recommended, none on the front. Master clinder is in original location higher than all rotors.
6. Bench bled master cylinder before installation, definitely no bubbles.
7. Bled probably 2-3 gallons through entire system using pressure bleeder. Even tappped on all rotors while opening the bleeder valve with pressure bleeder keeping pressurized fluid flowing out.
8. Tried 2 separate, made in the USA brand new master cylinders.
9. Swapped out residual valves in case the first one was defective.

I still get a low pedal on the first stroke. I noticed that the dual rear calipers do not keep the pads up against the rotor. I mean there is enough play to rattle the pads back and forth by hand in their respective slots. This supports my theory that the rear caliper pistons are retracting somewhat causing my first pedal stroke to have to refill them which is why it goes down further on the first attempt. Then they feel good unless you wait a few minutes, then it starts all over again.

Has anybody ever tried a 10lb residual valve on dual rear calipers? I know, it is meant for drums with the heavy springs, etc. But the question is, does it cause drag in a dual rear caliper setup? If not, maybe that is what is needed to keep the pistons more extended and the pads closer to the rotors?

Also, even though my front brakes are OEM factory set-up, would adding a 2lb residual valve to the fronts make a difference, or is my problem with that play in the rears?

I'm hoping somebody has tried a 10lb on a disc brake application so I can get some idea if that can be done.