'87 master cylinder question

[garystocker]

I am losing brake fluid at quite a clip but no leaks are detected at the calipers or wheel cylinders and no leak is obvious at the master, though I suspect the master is to blame.

Question is, besides the brake lines for the front & rear that connect to the master, there is one brake line on the bottom of the master. What is this for?

I cannot bench bleed the master with the line on the bottom plugged as the piston won't move in that case.

Once in the car the pedal remains soft since I can't seem to bleed it properly on the car.

Any tips?

 

[Mustang5L5]

The line on the bottom is the front driver's side brake. It recieves direct pressure off the master cylinder, while the front pass brake and rear brakes go through the prop valve through the other 2 lines....silly design, i know.

One place to check for leaks. Unbolt the MC and pull it forward. Sometimes the seal for the piston leaks and it leaks out the back of the MC and down the front of the booster. The booster will be rusted here if that's an issue, or sometimes it leaks inside the booste.r