What Dave Walsh said about disconnecting the caliper and rotate it so the hose is at the top. Also make sure to clamp the piston so it can't pop out and make a mess in the next step.

That next step being to POWER bleed the system! I use a small modified pump up garden sprayer, but a pressure like this would work too: https://www.amazon.com/Motive-Produc...4HD4ZPCTR0A800

On some gliders, with a small reservoir, I've installed a Schrader valve,
drilling out the vent hole (remove the valve and drill a small hole in the cap for the vent). A tube is connected to that and run to a glass container so you can monitor the results.

To avoid a mess make sure all connections are secure (you can wrap a couple of turns of safety wire around the caliper bleed screw to make sure that doesn't pop off. While bleeding, operate the master cylinder a few times.

When the problem is with the cable, it's often the outer sleeve of the Bowden cable. If there's a too sharp bend in it, the windings can tend to climb over each other effectively shortening the housing length and putting slack in the inner cable.