How do I isolate the leak?

Did you try checking for air, as I described,, could save a lot of yakkin, and save you from chasing your tail, - and once eliminated, would help us, - help you.. ??

"to check for air after,.. with m/c cap on,, have friend pump 2 or 3 times and hold down,,, remove the cap,, have pedal released as you watch fluid levels,... any "geyser" ( fluid spouting well above the surface) is compressed air, "springing" back,, front or rear,, -- -

-with the exception that,- if the rear brakes are outta adjustment,, the return springs can cause a small fluid return from the rears.. "

- this can be resolved by setting the emergency brake..

For over 30 yrs. been checking that way, works in my shop..