Help with brakes

[Panamaniac]

Hey guys,

Going to start with the backstory:

Car has OEM brake calipers, stoptech brake lines, and oem brake pads.
No ABS module is present and it has ChaseBays brake booster delete with a proportioning bias valve.

The car was bought with a salvage title. The car didnt break when brought by the Tow company. We had no pedal pressure at all. The rear lines were cut, so we replaced the rear lines with our custom lines. System was flushed twice.

After this we had some pressure, but still the car wouldnt brake. We had to use the E brake to stop the car will testing at 10 mph.

We bought a replacement brake master cylinder piston, installed and flushed twice the system. The pressure was better on the pedal, but still not hard rock. The car would brake, but would take longer than normal to make a full stop. ie @ 20 mph would take 5-6 meters to make a full stop.

So we went and open up all the brake calipers, checked everything and did maintenance on the pistons. This gave a better feel on the pedal, but still not rock hard. While testing we manage to lock the tires once when going at 30 mph. The problem is only once... after that it goes back to taking 5-6 mts to a complete stop and we couldnt lock the tires anymore.

What else am I overlooking? or should I just go back to OEM brake booster and MC?