The pistons in a brake caliper depend on the distortion of the D shaped seal to return them to the brakes off or resting position. (Dunlop/Jaguar tried to address this working clearance with the pad). When changing pads on these cars great care should be taken not to push the pistons back too far in the caliper to get the old pads out. You should just push them back sufficiently to release the old pads. If you don't do this, stiction of seals on the caliper bore can cause the piston to move forward but not bring the seal with it down the bore surface & cause the piston to over return when you release the brakes. You then need more fluid to bring the pads back in contact with the disc (rotor) which gives you a low or mushy pedal.
Now in fairness to both Dunlop & Jaguar they recognised the issue & have a little pin & sliding bush arrangement at the back of the piston to try & obviate this. Unfortunately it seldom actually works & has been dropped from more modern caliper designs.
There are two ways to fix this - one laborious & the other risks popping the pistons out of the calipers so be careful.
If replacing all pads does not fix the problem - then you need to remove all pads. Make up some wooden or similar spacers about the thickness of a brake pad backing plate. Insert these or old backing plates with no friction material on them into the calipers & stomp on the brakes to move ALL the pistons forward in their bores. Then push back gently until you can just slip the new pad in & you should should have cured the over return.
The alternative is to strip & dry all fluid out of the braking system. Reassemble the calipers with pads, shims, pins & all. Blow the pads against the discs with compressed air & then bleed the braking system.
Caliper Over Return is more common than many realise even on modern cars.
Another issue we had with my S Type's brakes. After correct bleeding from longest to shortest line we would have a high solid pedal that now & then would just go to the floor. Investigation showed that on occasions the Master Cylinder was just returning fluid to the reservoir instead of pressurising the system. Suspect inlet valve.
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The fluid inlet valve was obviously suspect but why. (Valve piston 11 in diagram & inlet port in the end of the cylinder).
After examining a number of rebuild rubber kits from different manufacturers it became evident that the new rubbers were very slightly short when compared with the original part. We could not find a single rubber to do the job. So we machined the end of the valve piston to achieve correct rubber protrusion & the brakes are reliable.
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