Hi Rob , here's a reply i sent to the list some time ago , hope it helps.
Brake pressure loss which feels like air in the system ,can be due to
the caliper seals grabbing the piston and pulling it back further than
normal when the brake is released.
I had this problem on my GTS front brake .
If i bled the brake, one or two weeks later the lever would be soft again .
My old seals looked ok , but when i changed them the brake was back to
normal .
I also cleaned the grove the seals sat in, because you can
sometimes get a build up of old dried up brake fluid behind
the seal ( looks like white powder when rubbed ) which doesn't
allow the seals to seat correctly.
The other thing to check is that the master cylinder is returning
completely to the end of its stoke.
Grab hold of the rod that comes out of the master cylinder ,
and see if you can feel any slack when pushing it up and down
( rear brake only ) , there shouldn't be much movement before
you feel it contact the piston in side.
Also check the seal between the caliper for damage etc (sent in by another
lister ).
One more thing to check is that the disc isn't warped
atb Nick T
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