Wilsons Sandbox

Argh, my brake fluid is leaking (well, more like seeping) from my fluid reservoir lid. It was fine until I opened it up late last week to have a look at the fluid level. It's seeping out from around the lid and one of the screw holes. What went wrong? I opened it up again this morning and wiped the sealing surfaces and closed it up again, but I don't know if that's gonna work. I'm leaving on my cross country trip with the bike this Wed. Is there any problem with topping it up as I go along? Also, the manual says only use the same fluid with what's already in there. Any problem with using something off the shelf to top it up with? I'm not ready to admit I have to postpone the departure date for my trip because of this. I'll form-a-gasket it before I do that. Any info/tips appreciated!!! Karl