Certainly right to suspect the rubber hoses; that said, it's quite common for the rear stub steel lines (go from the trailing arm pivot, where the hose is, to the wheel caliper) to get crushed during service. I've outright replaced mine - easy to bend up a new section of pipe. So check those hard lines for damage.

I don't quite understand the concern for running more than 20-30psi through the system; brake systems routinely operate at 100bar (not psi!!), and it's possible to generate over 200bar at the master cylinder.

That said, I would not try to use compressed air to push through any blockage until first disconnecting both ends of the line! Sucks to just push the block further down the line!

I expect the car has ABS; though this shouldn't even impact anything, I assume you're doing this with the car turned off?