OK, lets back up a bit and find out why the master cylinder and booster were both replaced. Gwaan83 would you mind letting us know please.
Now on these cars the brake system is very simple. Starting with the master cylinder (will skip the booster and brake pedal) the fluid goes through two lines down to the brake modulator valve (This senses if one side looses braking power). From there there are two brakes lines that, one goes to the front calipers, and the second goes to the back of the car just above the rear axle, then a rubber hoses, one on each front wheel that leads to each caliper, and one at the top of the rear axle which turns back to steel lines that take it to the rear wheel cylinders.
If no fluid is being lost (other than the brake bleeding) and no air is in the system then the master cylinder is bypassing inside itself. Or you could have a bake hose that is swelling up but most times they will burst if that weak. At this moment I am still suspicious of the master cylinder being your problem. I know you stated that you have bench bled it, but for most people that is not an easy task to do. What I do is put the new unit on the brake booster and have a helper push the pedal for me as I keep both my thumbs over the brake line ports. Have the helper push slowly and retract slowly so that all of the air is pushed out of the unit. This will make some mess but is very effective in removing all the air from it. Many times I then hook up the brake lines and really don't need to do any more bleeding of the system.