OK, the brakes feel fine most of the time but it's losing brake fluid. Had to refill the reservoir in one day.

With the engine off, the brakes are nice and hard. With the engine running, the pedal will slowly go to the floor.

It's a new master cylinder so it shouldn't be into the vacuum booster. Looked at all the rubber lines to the calipers and they don't have any signs of fluid leak nor do they balloon when the pedal is depressed. No fluid on any caliper or rotor. No signs of brake fluid under the car, but it leaks tranny fluid and engine oil so it could be masking the brake fluid.

Suggestions where this leak could originate?

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There is a very fine line between "hobby" and "mental illness."
FrankenBenz: '87 560SEC with an '02 cammed LS6 and custom 4L60E.
Bling: '87 560SEC, ported/polished, lowered
Lisa: '87 560SEL, lowered