It's exposed to atmosphere but it is not straight down. There is a baffle in the opening that prevents slosh induced leakage.I'm ready to install my Tilton, but I do have one bit of reservation - the cap on them has a tiny vent hole that is just a straight hole down.
I would suggest that flushing brake and clutch fluid once a year is pretty easy and might in fact be a good thing to do. My clutch circuit fluid was nasty after just a thousand miles. Brake fluid is cheap...
1. I've fought Wilwood M/C for years of slow leaks
2. Porsche plastic reservoirs have similar hole and I've had similar concerns
3. Not rare at all for us to see fluid pushed out during track use and fluid expansion IF not slosh of these Plastic lids. To the point that many run over flow bottles or drain lines through trunk floor.
4. Alex......If your going to play with the big boys and run a BBC in a PcarYour going to have to invest in a power brake bleeder and just flush system. If your not needing to flush brakes after a track day, maybe your not driving hard enough
Personally like the ATE typ200 Blue as best for bang your for buck.
Alex, not sure what to tell you. In the picture above of my setup you can clearly see the fluid level. Have you tried pointing a flashlight at it from the side?
hold the beam directly against the side case - it's like x-ray vision.....