most of the time,the pedal connnection to the rear-brakepump tends to wear .
then you can move the pedal up and down without activating the brakepump.
i solved it by making the oval wearhole a round one and fit a homemade bushing in it
i did it with 2 out 3 k5,s ,never issues with that again,it this your issue too ?
or am i talking bout another thing