> I've also heard of brake hoses collapsing internally and causing this.
> They create a sort of check-valve effect -- the high pressure when you
step on the pedal can easily push through, but then the fluid has
> difficulty returning to the master cylinder.
>
>  David Brodbeck, N8SRE

You are quite right. I have found this condition quite often in older
vehicles. It could be caused by the natural deterioration (aging) of the
hose material or by the pulling or "yank" on the brake line when the
calliper falls during brake work.

The outside may look okay, but look near a crimp fitting, you might find a
bulge. The hose may also have a soft, "mussy" feel to it. You won't always
see any signs of internal damage on the outside of the hose. The damage is
all on the inside and a flap or "check valve" action can occur.

To make a quick check, remove the caliper from the vehicle and apply
pressure by using a "C" clamp (you may have to bridge duel pistons) making
sure your are pressing the piston evenly in.

If you meet a great amount of resistance or can't move piston,  open the
"bleeder." Now reapply pressure, if it sprays out the bleeder and now you
are able to easily press the piston in, it is most likely that the brake
hose is damaged internally.

Replace hose and retest. If you still are unable to "force" fluid back into
the master cylinder, you may have dirt in your lines (system) or your master
cylinder is bad.

Paul