Maybe some one knows for sure what parts kits are made from. looks like EDPM would be a good choice
BTW...You do know the square cut is used to allow the piston to "pull back" when pedal is released.
( I wonder if the quad would give more pull back?)
(intersting, i just googled and used the best looking compatibility chart, however I noticed it shows SBR excellent for mineral oils where I thought it was poor)
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