Hal,
When you say that the mechanical linkage is at the limit, do you mean
as long as possible or as short as possible? You are having classic
symptoms of insufficient master cylinder piston return. When the piston
comes all of the way back, the hole between the cylinder and the
reservoir is uncovered and fluid can transfer between the two. If the
linkage is too long then the hole stays covered and fluid can't get back
out. It can still get in because of the seal design so the brakes just
pump up until they are solid. Usually adjusting the linkage to leave
some free play fixes the problem.
If you already have some free play, then the hole could be plugged be
debris. You didn't mention what type of brake fluid you use but
silicone fluid sometimes swells the seals sufficiently to keep the
transfer hole closed. Solutions for this would include shortening the
master cylinder piston by one millimeter or so, putting a 1 mm shim
between the master cylinder body and the stop plate, or changing to
Castrol LMA after replacing the seals.
Another slight possibility is that the flex hose to brake slave
cylinders have degraded and collapsed. These can act as one way valves
and cause the brakes to drag and lock as you drive and the brake fluid
expands with heat.
Good luck,
Bill
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