Should I replace the master cylinder or the slave cylinder?

[abinderup]

For the past year I've had a problem with air getting into the clutch hydraulic system on my '92 Exp and making shifting difficult. When I bleed the system everything is fine again for 2-3 weeks, but it always comes back again. From what I've read here, and what I've been told by the two mechanics I've talked to, it seems like the problem is with either the master cylinder or the slave cylinder, with the most likely culprit being the slave cylinder.

Given that the slave cylinder is much more expensive to replace I'm thinking about taking my chances and replacing the master cylinder first in order to rule it out as the cause. Is this a waste of money? Should I just bite the bullet and do the slave cylinder? Please help! I'm getting tired spending my evenings with brake fluid dripping in my face.

Thanks.
Adam B.