Here's one that might stump you

[dave]

My thought is this: There's a very small hole inside the master cylinder that is clogged, or partially clogged. You need to completely drain the reservoir to see it. (There might be two holes, I don't remember). Once drained, use a small wire to poke into the hole and dislodge any contaminate, and high pressure air to blow everything out. I would either do the work yourself, or find another mechanic to perform the work. Whatever you do, don't continue to ride your brake with a front brake that you can't rely on. Good luck.