I believe that it will be same as the A90 Atlantic, as well other models, and the shimm is required to allow the fluid to flow past the washer, then into a step/rebate around the inside diameter of the seal. From this stepped/rebated area,the fluid then flows into the centre of the piston, which on the Atlantic is through two small holes just back from the end of the piston, with these holes aligning in the stepped/rebated area of the seal.
In the at rest position, fluid seeps through from the internal reservoir of the MC, then as the pedal is depressed, as soon as the two holes in the piston pass through the seal, pressure immediately builds, and within the MC, this firmly seats the inner seal around the piston and against the outer diameter of the seal in its bore.
When the pedal is relaxed off, tests I have undertaken on the bench shows an immediate backflow of fluid out of the inlet pipe, (clear plastic tube for the test), as the pressure is released from braking. With there being no washer under the seal, I would think that this is the reason for your problem.
Having driven my coupe for 300,ooo miles, I discovered this problem, but not because the shim washer was missin, but because there was no step.rebate on the inner diameter of the seal, and the two small piston holes were blocked by the straight inside of the seal.
Hope this assists.
Regards-Peter.
Australia.
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