This does not follow. I can think of several dozen times over the course of 30 years where I have put replacement caps on used masters that continued with many years of useful service. A couple of those have been on my own Cruisers.
The Elder Statesman is Absolutely Correct , REplacing a Master Cyl. Cap
Clutch or Brake , is a Normal Age & Time Interval Occurrence .
Lets all Remember & Recall , they Rot From the Outside inward , 9 times
out of 10 , this would imply a outside influence such as PCV gasses, CO,
NOX, maybe HC too ? certainly engine bay Temp. fluctuations , and Fluids
of various Kinds making contact to caps over a course of years , is common
place , i personally have NEVER seen one Rot or Crack from Inward to Out ?
BUT, ..............................
None Of this Really Matter's to a Level of Concern that Requires a
any further Relevant Debate any Longer Since :
VintageTeqParts.Com & Myself
WILL , BE Providing Fellow Member @Lifelong40Fan a Professional's
Sample's 3pc OEM TOYOTA Parts Reserviour Cap Kit with No Cost
Involved , including 2 Day Priority Mail Shipping to his Shop Door , & 1
Pint OEM TOYOTA DOT 3 Hydraulic Fluid . 
Lets help Preserve both His Master's by Using Modern Materials &
Techniques with a Purist Based Mind-set approach ...........
PM sent ...........
