Beat me to itIt could be leaking past the master cylinder's rear seal, into the brake booster ... .
That wouldn't explain his loss of brake fluid, however.Is the new master cylinder push rod and yoke the same length? If not it may need adjusting to take up the brake pedal freeplay
Take it to an independent mechanic. Too dangerous (potentially).I have been putting brake fluid in alot lately. I havent found a leak at all. My brake pedals go all the way to the floor.