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When you wish to push the pistons back in to fit new pads, you cannot, since it is held in position by this one way mechanism. However, you can rotate the piston to wind it back down the spiral, but you must apply pressure as you rotate the piston, and since the frame stops you using a conventional wind-back tool it is a little more difficult, particularly when on the car.
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Roy
I finally had to do this work on the rear calipers. The reason being that it would fail the biannual MOT (Syn, in Danish), because the hand brake was unable to hold the car to the point of stalling the engine. That is about the braking power it needs to hold a car on an 18 degree sope as the law calls for in Denmark.
Part of the work consisted of replacing the outer hand brake cables from the caliper to the chassis. Last year I replaced the inner cable with a stainless one. (Still looking for stainless outer cables)
the old ones were so rusted that they compressed when pulling the hand brake.
The rest of the work, and the hard part, was to screw the pistons back in, to fit the Green Stuff pads I skipped last time. With help from Jesper, who held the caliper with a huge pipe wrench and applied pressure to the piston with a long 19mm wrench, I was able to turn the piston and it actually retracted. The left one took a lot of pressure and many tries, while the right one moved fairly easy, using the same tecnique. I naturally had the dust seals removed while doing this.
With the new pads installed and the brake applied to seat the piston, I could adjust the hand brake to the correct tension. It now can stall the engine, if you try to start in 1st gear with the brake pulled tight.
It has never worked this well.