First- from what you're saying, you step on the pedal and it holds, right?
Old hoses can close up- but this usually manifests by having the brakes stuck on... But that's where I would start- check the hoses... Maybe just crack the lines in then out... Then at the wheel cylinders... Pistons could be stuck in their bores- but then they would probably leak...
If you remove the flex lines you should be able to blow through them, not much though...
Karl