Mysterious Brake Fluid Loss

[xdarkxfirex]

I have done quite a bit of work on the brakes on this car. Replaced several aftermarket wheel cylinders that went bad quick and get better ones. Also replaced a master cylinder with a dorman last year and put the old one on because it was fine, and dorman started leaking at where the circlip is at the booster.

Recently I notice a slight drop in fluid with no leaks. I took my master cylinder off, and found fluid, most likely from the old bad one, but none where the circlip and seal were. I mark the fluid level with a marker. Right after I filled it above the max line, probably a 1/8" from the very top. Drove around 25km, it dropped about an inch. Marked it again, drove home, then another 25km and its exactly where it is.

Could this fluid loss be from overfilling or settling where the catch basket was?