A manual is your best friend...
Regarding that silver washer, see page 10 of the manual, its not shown there but the washer goes in between the piston seal (Figure G) and the reg spring (Fig.H).
It basically prevents the spring from scratching the surface of the piston seal.
What are your board settings?
Drop off might mean your noid dwell and ABS dwell is too low and/or bad lube.