Forums General Counties Cars Discussion Somerset master cylinder

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    The original master cylinder had a shim washer under the larger seal

    the washer was dimpled to allow space between the seal and the master cylinder body

    i have bought a new master cylinder but it does not have this shim

    i am having trouble with the brakes locking on

    Can anyone explain the purpose of the dimpled shim or any other cause for brakes locking up

    many thanks

    Geoff Levear Oregon USA


    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.





    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.




    Many thanks for your observations and description

    i will have to take a closer look into the master cylinder for the holes you describe in the piston. I don’t recall seeing any but will look again next time I dismantle it.

    I’m not quite clear on your description of the step/rebate around the inside diameter of the seal. If you have a picture or could sketch it out , maybe you could email it to me.

    a picture is worth a thousand words

    thanks again for your effort to help I was at the limit of my knowledge and hoping for as much help as I can get from all the club members as Austin knowledge is thin on the ground here in Oregon

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