Brake fluid level dropping but very slowly

The brake fluid level in my 2007 V6 4x4 Tacoma used to drop from the MAX to below the MIN mark on my brake fluid tank in a matter of 3-4 days. I thought there was a hole/leak somewhere on the plastic tank that is mounted on top of the master cylinder.

So I bought a new plastic tank and replaced it. I thought it fixed it but the level dropped again, however, it took much longer to lose the same amount of liquid (about 2 months).

When I was replacing the tank, I noticed it was dispensing the fluid into the master cylinder via two tubed studs in the bottom of it and between each and the socket on the master cylinder there was a rubber O-ring gasket.

In retrospect, I am thinking it was not the plastic tank that was broken (which is really so simple that it would be hard to break) but the gaskets were leaky because they may have shifted location due to motion and friction (like every other part really).

I am posting this to inquire about the likeliness of this suspicion and whether that is a common cause of leakage of fluid between the plastic tank and the master cylinder. Also, I am having a hard time finding the gaskets at my local Toyota dealership. Do you know the name for these gaskets so I can order them online?

 

more likely a leak at one of your rear wheel cylinders or front pistons.

 

I was thinking the same but the level doesn't drop past about 1/4" below the MIN mark (it stops there). If your assumption were correct, shouldn't it keep dropping? My explanation why it doesn't go below much is because there is less pressure with less liquid in the tank after some has been leaked out so now the bad gaskets can sustain it.