Conversely if the front brake circuit fails - as in the OPs case you notice it and quickly. Loaned my Saab to my brother and he brings it home one night saying " I think there is something wrong with your brakes" Well a front flexible line was slashed - and the there wasn't much fluid left nor braking , can't believe he even made it home. Moron. Took nearly a week to get the part as nobody local stocked them - extremely rare occurrence.Hi Folks,
This is a summary of FMVSS 105 as it is way too complicated to post here. This is from the Haynes manual.
"By 1967, it was mandated by the Federal Government that all vehicles use a dual-brake master cylinder, with separate circuits in case of the failure of a line or other issue. The resulting dual circuit master cylinders typically have two separate chambers that separate the front and rear brake circuits, though some still are split diagonally.
This type of system prevents the total loss of braking action in the event of the system being compromised. The brakes still work even in the event of one seal in the master cylinder going bad, or a pressure leak in the system.
Either systems will stop the vehicle with only one circuit operating, and prevent an accident, though stopping distances are much greater. More than 75% of your braking is done with the front wheels, so if the rear brakes fail you may not even notice."
Ranger Vehicle Engineering
Ford Motor Co. Retired