The difference is that with the bleeder valve open, you aren't
developing any pressure across the piston seal. With the valve closed,
you can develop pressure and the seal leaks.

Steve H

"G.R. Patterson III" wrote:
>
> snh9728 wrote:
> >
> > I didn't mean an external leak; I meant a leaking piston seal. The
> > piston moves forward when you push the pedal, but the fluid just leaks
> > past it to the other side of the piston. The fluid is still in the
> > master cylinder; it didn't squirt out into the world.
>
> If that were the case, I would think that nothing would come out the bleeder
> valve when I opened it up, but I do get some fluid out of each valve. About 25
> pumps with a valve open will draw the level in the master down about half way.
>
> George Patterson
> If a man gets into a fight 3,000 miles away from home, he *had* to have
> been looking for it.