Re:
I think that your brake adjustment problems are not exclusive to Grafton brakes
Over the evolution of the cantilever brake there has been a shift to ever greater mechanical advantage
in the brake design . This means that for a given brake lever pressure the pad pressure has increased
This change has been achieved by looking at the basic geometry of the brake lever pivots and
more importantly increasing the brake arm lengths while reducing the profile ( for clearance )
The result has been a greatly increased pressure on the brake pad causing the pad to deform temporarily
as it touches the rim . This deformation is felt as a spongy/squidgy feel at the lever
To quote Sheldon Brown " brakes with a rock hard feel may seem nice on the workstand , but when it comes
to making the bike actually stop , the spongy setup will do the job better with less finger pressure and
greater margin for safety in wet weather "
You have several options to solve this ' problem ' firstly reducing the mechanical advantage of the brakes
by increasing the length of the straddle cable . However there is not much room to do this in your setup .
Secondly you need the hardest ( non squidgy ) brake pads while retaining a high coefficient of friction , and finally if your
brake pads are of the post fitting type , move the brake arms further apart to change the basic geometry , they will
no longer be quite so low profile , but will be slightly less efficient which will help . You are looking for marginal
losses here !
There is also a possibility that the seat stays are moving slightly as mentioned above
