what is the seal behind the brake resovoir called?

there is brake fluid behind the brake resovoir running down my firewall, what is this seal or gasket called so i can get it replaced? Thanks for the info in advance

Sounds like the master cylinder is leaking. Needs a new one.

guess i'll get a new one next paycheck. It should be ok if I keep the fluid up. I think I'll just take it to a dealership and let them change it. Thanks for the info

Yeah theres no gasket on the firewall. Sounds like the piston is somehow leaking. You are most likely still going to be taking air into your lines, so I wouldn't drive on it too long like that. It's not a hard job, but it's time consuming ( mostly bleeding everything )...

not the firewall, my bad, it's the brake booster. I won't have enough $$ until the end of july. Hope it holds out untill then. I think it will