"All o-rings in the brake and master cylinder assembly are buna nitrile and are not compatible with automotive glycol based brake fluid. ONLY red aircraft fluid (Mil-H-5606) or other suitable petroleum-based or silicon-based fluid should be used." 

 

That would be a reason, except that most automotive brakes use Buna-N seals! Some high temerature brakes use EPDM (viton) for increased temp resistance, but Buna is common.

 

Bill Jepson