Being able to pump up the brakes is not normally an indicator of a defective
master cylinder, it's more commonly an indicator of maladjusted brake shoes.
It can also indicate a problem with the calipers.
In any case, you're first push on the brake pedal pushes the brake components
out to the drum (or against the rotor). The second pump happens faster then
the brake components retract. Hence the pump up. And, hence the
repeatablilty when you give the brake components a chance to fully retract.
A bad master cylinder will not pump up.
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