crack the hose fitting loose, on the end furthest from the caliper, then give it a squeeze. if it doesn't flow, the inner liner has collapsed, and is forming a seal preventing backflow, as mentioned by stumpy. if it flows freely, tighten it up, and move back towards the proportioning valve, and crack each fitting, until you get no fluid. the hoses are known to delaminate inside, causing a one way flap.