The brake fluid reservoirs for the 928 are priced like gold. Maybe the reservoir itself is unique to the 928, but the rubber cap, I bet, is used on many models of ATE reservoir from the 80s/90s. How to go about finding out? Dunno. Measure the cap or OD of the surround for the cap. Visit some forums and/or junk yards.
Hanlon’s Razor: Never attribute to malice that which is adequately explained by stupidity.
Heinlein’s Corollary: Never underestimate the power of human stupidity.
The Reddit Conjecture: Sufficiently advanced stupidity is indistinguishable from malice.
Worf’s Razor: Never attribute to stupidity that which is adequately explained by laziness.
Worf’s Identity: Sufficiently advanced laziness is indistinguishable from stupidity
Worf's Law: Once you've mitigated risk from stupidity and laziness in your endeavors, failure is usually the result of insufficient imagination.
My 928 Inspection Guide