So, last weekend I went through the task of putting in a new brake
master cylinder on the GoldBrick, my 86 passenger Syncro. It had
classic symptoms of a failing master cylinder, so that was an obvious
fix. Flushed the brake fluid, bleed the brakes, and all is happy. Got
it through Virginia State Safety inspection and its good for another
year (strange thing is it probably wouldn't make it through a Maryland
inspection due to some of its rust...but that's a tale for another
time.)

On my Westy Syncro, however, the brake problem is different. The
symptoms are as if I had air in the brake lines, but I've bled the
brakes (and flushed the fluid) all the way around. Essentially if I
step on the brake pedal and pump it multiple times I'll get braking
action. However, occasionally, when I hit the brake pedal quickly (and
release) I'll hear a pneumatic flapping sound (like air going through
a diaphragm.) When that happens I get excellent brake feel, where
you'd expect it, about 1 inch after depressing the pedal.

Most of the time, however, its down near the bottom of the floor.

I've been pondering this and considering that it might be the booster,
but it can't just be that. So I'm going to try and replace the master
cylinder on this vehicle too. Unless folk have other suggestions as to
what it might be (discs all around, SA big brakes up front, Smallcard
(Audi) discs in back...all done within the last 6 years, and I've
replaced the pads up front, once since the big brakes were installed.)