If there is residual air in the system, when you pump the pedal, it comrpesses the air, and when you release the pedal, it can push fluid back into the reservoir quite violently .
To test, with emergency set, with reservoir caps on, - have someone pump brake to hardest, remove cap, have pedal released, there should be next to no return to the reservoir, any large return, or " fountain " of fluid is likely air . With shoes against drums, springs can't cause return .
CAUTION. . Brake fluid is excellent paint remover,. Rinse with water immediately. ! ! !
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