Since you stated you have all new , we're going on the assumption it's new everything and none of the old stuff is still being used The valve you refer to is the residual pressure valve, which keeps a bit of fluid in the lines to keep the shoes from pulling too far away from the drums. This is not used in the front disc circuit but has caused some grief for those who have replaced the front drums with disc and not taken the valve ball out of the system, Usually they are in the outlet ports of a drum/drum MC. You've bled everything You clamp off all the lines and your MC has good pedal pressure. You unclamp your front brakes and have no pedal pressure and your front brakes don't hold. Sounds like the MC is not supplying enough volume of fluid to move the pads against the discs. This could be due to a MC that is not compatible with the front brake calipers. Just doesn't have the physical ability to supply the volume required to move the cylinders in the calipers. Or, the MC piston is not being moved it's full travel thus not moving the required volume of fluid. It would take very little travel of the pedal and very little volume of fluid to get pressure in the front lines with them clamped off compared to the amount of fluid it takes to move the brake caliper pistons. Is the MC a one inch dia. or larger piston? is the brake push rod in the correct hole in the brake pedal lever? Is the push rod the correct length? Or there is still air in the front circuit. Or the system is not connected, plumbed, correctly. Or you have defective parts. Or you have parts that are not compatible with each other. You should have a metering valve and a proportioning valve. Or a combination [metering, proportioning, differential pressure] valve. As the front pads are always slightly in contact with the disc they are in the position to activate with little increase in fluid pressure. The rear shoes on the other hand take more to activate as they are not in contact with the drums. The return springs pull them slightly away and it also takes more fluid to move them than the front discs. The metering valve doesn't allow any pressure to the front circuit until the rear circuit builds a small amount. This is called threshold pressure. When the threshold is reached it allows pressure to the front system. This causes the rear brakes to just start to engage before the fronts. It helps to keep the car straight during braking. If the metering valve is defective it may not be letting pressure or only a reduced pressure into the front circuit after the threshold pressure is reached. The proportioning valve reduces pressure to the rear brakes. It takes less pressure to lock up the rear brake than the front brakes. As the car de-accelerates the weight is transferred to the front requiring more braking power at the front and less at the rear. If this valve is connected backwards it will be reducing the pressure to the front and not the rear. The pistons not moving in the calipers due to a mechanical problem could cause lack of brakes but the fact you have no pedal pressure would most likely discount that. The fact you can get pressure with the front lines clamped and not with them unclamped points to air in the calipers or just not enough fluid supply. If you have everything connected correctly and there is no air in the system, I would say the MC is defective or you have mis-matched parts. If you do the Flintstone thing cut a hole for the passengers, every bit helps. But, I could be wrong.
