Brake Pedal going to floor - only front brakes working

If you don't have a restriction in the lines theres a valve called the proportioning valve, it delays braking to the rear so they don't lock up before the fronts have a chance to grab, in vehicles with front disc braks and rear drum brakes. Since I don't have a clue what type of vehicle, year or make your going to have to supply me with more information. Theres also a metering valve and a pressure differential valve. Depending on the vehicle these three vlaves could be combined into one unit. Hope this helps.

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These valves have a very low failure rate.

If you see brake fluid leaking external to the system you need to fix the leak.

If the rear brakes are leaking fluid and it's a drum application, it's probably leaking from the wheel cylinder or possibly a brake hose.

Check the brake fluid level make sure it's topped off.
It wouldn't be a bad idea to flush the system after you have identified the leak.

If no external leaks are found, there could be a problem with the master cylinder internally bypassing as well.

Check the brake system for external leaks, let me know.