Could have been a bit clearer regarding the shim washer and the seal in the last comment of my earlier response. Whilst fluid will seep through without the shim washer, it is essential in allowing a very fast pressure relief when releasing the brake pedal, for if the seal does not have the step/rebate around the inner rear diameter, and it is a tight fit onto the piston diameter, both against the rear of the seal and not having the rebate in the seal, allowing blocking the holes near the end of the piston, I believe that this is what is allowing the brakes not to release properly, as the fluid cannet bleed back through the holes in the end of the piston into the seal step, and under the shim washer that has the dimples on ether side to allow ease of return flow.
Hope this is is clear enough. As a point to note, this is not the first time I have heard of binding brakes, with a number of reports that the BN 1 & 2 model Austin Healeys having suffered from this problem, and they have the same master cylinder as the A90 Atlantic.
My comments are based on your mention of the shim washer, indicating the master cylinder you are referring to is the same as on the Atlantic. These have an internal inlet reservoir, separated by an inner partition in which there are 6 holes, probably being about 1/16 inch diameter. The dimpled washer seats over the holes behind the seal, which has the step/rebate around the inner rear diameter of the seal. the holes near the end of the piston align with this step in the seal to allow fluid to flow through. In addition to the step in the seal, I guess one should also ask if your piston has the holes.
Hope this is of some help. Cheers-Peter.
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