Cherokee 180 soft brake - causes?

Jay Honeck wrote:

Our A&P hates bleeding Cherokee brakes, but it's more of a repetitive pain
than a really difficult procedure. He just climbs under the wing, holds the
line (or brake assembly?), and has me pump the brakes until nothing but
fluid comes out. It can take several pumps, and uses a bit of brake fluid,
but it works well. (I've not seen what he's doing under there, since I'm
inside the plane, but he's catching the fluid in a container of some sort.)

They can't be bled from the bottom up?