Low brake pedal?

[Ken K.]

I have tested the master cylinder and it does not seem to be bypassing. I
think we can rule that one out. I've thoroughly bled the brakes. I suspect
the rear brakes to be faulty. I occasionally hear a light *clunk* *clunk*
*clunk* as I come to a stop. I never could tell what was wrong from a visual
inspection. Any ideas?

Can a faulty wheel cylinder cause a low pedal condition?
I don't see any leaks anywhere inside the drums.
I read somewhere that a faulty booster check valve can also cause a low
pedal condition. How is this so?
I believe my check valve is fine since it holds the vacuum for hours or even
overnight.

I tried adjusting the slack out of the brakes and that did not help my
situation. Is it advised to clamp off individual brake lines to isolate
where the problem is?

Ken Kubotani