I don't know if this would work for the S4 spiders, but on my 78 MC rebuild and reinstallation I just bled the system from the output brake lines where they connect to the MC. On my car they are vertical and when they are disconnected only the MC dumps fluid of any consequence. I primed the MC with the tubes off once the MC was reinstalled on the booster by slowly pumping the brake pedal. A couple of catch cans and rags and extra brake fluid came in handy. I had someone press on the pedal slowly as I quickly reattached the brake lines. Once the lines were tight, I had my helper release the pedal. I then loosened one line at a time to bleed out any air and had my helper again press the brake pedal slowly. Once I was getting all fluid from the loose fitting I then tightened the line. I repeated this process per line. You will know you are getting it done right as the pedal regains its firmness. Its a messy process, but it works. I again stop on a dime.