I did put it from the outside. It has a lip/shelf on the inside, which the dime compresses an o-ring against, then a metal collar is pressed in to hold everything in place. It is held in the same way as the plastic window, the window had just disintegrated, so I used a dime as a replacement. The top part is not pressurized, at least not the same way as the master cylinder/caliper system. If it leaks, I will still have brakes! I wouldn't want to risk "rigging" something that is part of the pressurized system!