My guess is water ingress inbetween the mater cylinder and servo units.
Undo the 2 nuts and pull the master cylinder away from the servo, any signs of red rust in there is a problem.
I've seen quite a few where the piston in the master cylinder hasn't returned to rest fully. If this is the problem, you'll see that the piston is 1 to 2mm away from the circlip.
I've seen a few of these get hot enough to melt the wheel nut caps!
BTw, you don't need to touch the brake fluid system to check this, the pipes will allow enough flex to have a wee look inside.
the root cause is a perished O ring seal between the 2 units.







