>From Yodal@gnn.com:

>.  There is 
>an additional valve in the disc brake system whose nomenclature 
>escapes me at the moment and you could adapt the power assist for 
>the master cylinder but I didn't and had no problems.

One of the differences between hydraulic drum- and disc-brake systems
is the (what-i-call) rest-pressure. Rest-pressure is a slight over-
pressure that remains in the hydraulic circuit when it's in rest.
Rest-pressure is maintained by a valve in the master cylinder.

The drum brake system benefits from this slight overpressure as it helps
keeping dirt, water and air out of the system. The pressure will not
overcome the springtension on the brake shoes, the shoes will not 'brake'.

A disc brake doesn't have return springs. It has a thick rubber ring
between caliper and piston. When force is applied to the piston the rubber
ring deforms axially, when the force is taken away the ring wants to
restore itself and while doing that it backs the piston off a little. Just
enough to avoid excessive pad/disc friction. When the travel of the piston
is more than the rubber allows, it just slides through the ring, ie. it
adjusts automatically as the pads and disc wear.

It all comes down to this: the rubber ring in a disc brake cannot retrieve
the piston properly when there is rest-pressure behind it, there will be
too much friction when the system is in rest.

Brian.