Never ever use synthetic for brake fluid. It's a big no-no especially with ABS. The syntethic stuff swells the grommets and gaskets and causes leaks, hence your poor brake pedal feel.
Flush ASAP and replace with a good DOT3 or 4 fluid such as Wilwood or at a minimum Castrol GT/LMA.
If the synthetic stuff has been in there a little while already (as in more than 24 hours) you will need to replace ALL hoses, gaskets, grommets and the master cylinder and ABS pump (if equipped). OUCH, big $$$ unless you're a DIY and have LOTS of time on your hands over the holidays.
Wish I had better news.
