Graham,
Not sure if your theory of what the problem might be is possible. However, I
have two other suggestions.
A few months ago I was having problems with soft brakes. Bleeding helped a
little, but not enough. Two other things made the difference:
First, brake pads too tall. They were a tight fit in the caliper when I fitted
them, but stupidly I did nothing about it. When the pads were worn a little,
you would press the pedal and the centre of the pad would move towards the
pad, and press against it, but the top and bottom would jam in the caliper and
not move - instead the pad would bend a little. When the pedal was released,
the pad would spring back towards the original position. Over time, the gap
between the pad and disc got bigger and bigger, as the pad was worn down, so
more and more pedal travel was needed to get the pad to contact the disc.
Solution - remove pads (so badly jammed in I needed to remove the caliper from
the trunnion to get them out), grind a little metal off the top and bottom,
slip back in, brakes much better!
The brakes were still not perfect, so I suspected the 22-year-old rubber hoses.
These were all replaced with new steel braided items, and this made a stunning
difference to the sharpness of the pedal. Soft, old rubber was expanding to
take up most of the fluid being pumped down the brake lines.
Good luck, I hope you fix it!
Richard & Daffy
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