Is there a brake fluid-compatible thread sealant available? I'm suspecting a leak between the master brake cylinder and one of the fittings on a hard line connected to it.

Permatex makes this (but will it deteriorate w/ exposure to brake fluid?):
http://www.permatex.com/auto/autouh....&item_no=56521

EDIT: Just found this that might be more suitable (Permatex Pneumatic/Hydraulic Sealant):
http://www.permatex.com/products/ind...ulic%20Sealant

 

I wouldn't bother with sealant. The seal in brake connections ise actually made at the tapered seat in the end of the hardline. If brake fluid can even get to the threads then the seat is not sealing. IMO adding some sort of sealant to the threads won't really fix the problem because it doesn't repair the seal.

If you're careful not to get the sealant inside the lines I guess it doesn't hurt to try though.