A brake fluid reservoir must be vented in order to allow the fluid level to rise and fall without resulting in pressure changes.  The problem: air in contact with brake fluid is not good.  Brake fluid may absorb some of the air, forming microscopic bubbles and making the brake pedal feel "spongy", but this is not really a big problem unless you use "Dot 5" silicone fluid.  The bigger problem for anyone using regular brake fluid is that the fluid will absorb moisture from the air.  This moisture does two things: first, it causes corrosion within the brake fluid circuit; and second, it lowers the boiling point of the fluid, which can cause problems when using the brakes hard.

To minimize all these problems, brake fluid reservoirs are typically vented using one of two schemes.  The first is to put a simple hole in the cap, but in order to minimize the effects of the air being allowed into the reservoir, the hole is really tiny and path the air takes is convoluted.  The idea is to prevent the air from freely flowing through while still avoiding any pressure or vacuum to build up.  A small amount of air can come in or go out as needed, but it generally won't just breeze in and out.  The air inside will tend to stay inside, and since that air only has a finite amount of moisture in it, only a finite amount of moisture is available to be absorbed by the fluid.

The second scheme is better.  A diaphragm is provided in the top of the reservoir; once the cap is closed up, the fluid is underneath the diaphragm and the air vent is above the diaphragm.  If the fluid level goes up or down, the diaphragm goes up or down with it, and the vented chamber above the diaphragm permits this, but the diaphragm prevents the air coming in the vent from coming in contact with the fluid.  When the cap is closed up, some air may be left under the diaphragm and therefore in contact with the fluid, but this air will only have a finite amount of moisture in it.

From this explanation, it should be apparent that opening up the reservoir to check the fluid level every now and then is not a good idea.  That's why reservoirs are usually made of translucent plastic: so you can see the level of the fluid without removing the cap.

The Mitsubishi remote brake fluid reservoir has the diaphragm in the cap.  Unfortunately, in the original design, three notches were provided in the outer sealing flange of this diaphragm and three mating tangs were provided in the cap to serve as an anti-rotation feature.  The idea was to prevent the diaphragm from turning in the cap as it was screwed onto the reservoir.  Unfortunately, the three notches were the design's downfall; they permit the diaphragm to distort as the cap is being screwed down, and the sealing flange distorts and gets cockeyed and loses the seal.

You can easily see the distorted area at the left in this picture.  That distortion is defeating the purpose of the diaphragm, and allowing the fluid in the reservoir itself to be vented directly to the outside.  The venting of this reservoir is actually around the edge of the cap, via the two areas where the threads are missing.  This venting is supposed to get only to the chamber in the cap above the diaphragm, but when that sealing flange pulls inward like that, the venting can pass right through the resulting gap and into the fluid itself.

In other words, this defect changes this diaphragm type reservoir into a non-diaphragm type reservoir.  Not a big deal, it just means you should be changing your brake fluid more often.  What's that?  You've never changed your brake fluid?  Tsk, tsk.

Once the cap has been screwed on and left in this condition for some time, the rubber the diaphragm is made out of takes a "set" and cannot be made round again.  The one in these pictures is like that; you can straighten it out with your fingers, but as soon as you let go it returns to its distorted shape.  If you picked up a Mitsubishi reservoir in the junkyard and the diaphragm has this problem, you're going to need a new diaphragm.

Mitsubishi's fix was simple: do away with the anti-rotation feature.  Who cares if the diaphragm rotates in the cap as it's being screwed on?  The new diaphragm is part number MB895813.  Here's a comparison of the original diaphragm with the replacement:

The small notch in the edge of both diaphragms is the actual vent that permits air coming past the missing sections of threads into the chamber in the top of the cap.  It doesn't matter in which clock position this notch ends up in the cap, since the air can easily travel circumferentially around under the bottom thread in the cap to get to the notch.

To go with the new diaphragm without notches, you're gonna need a cap without tangs.  You can take your trusty Dremel and carefully cut the tangs out of the old cap.  You can see two of the tangs in this picture:

Actually, all three tangs are different; one is an anti-rotation tooth leaning one direction, one is an anti-rotation tooth leaning the other direction, and the third is just a rectangular tang.  Doesn't matter, they all need to get cut out.

Or, you can take the easy way and just buy a new cap from Mitsubishi along with the diaphragm.  Or forget the junkyard parts and buy the entire reservoir new from Mitsubishi, part number MB534534, which will include the upgraded cap and diaphragm.

Several cars built by Mitsubishi have been sold in the US with the Dodge badges on them.  Apparently somewhere along the line Dodge became aware of the shortcomings of the Mitsubishi brake fluid reservoir cap and diaphragm, and rather than wait for Mitsubishi to address it, they introduced upgraded parts of their own.

Walter Acker IV found such a diaphragm assembly in a Dodge D50 mini pickup truck.  The Dodge diaphragm is a two-piece design, a rubber diaphragm with a hard white plastic rim that snaps onto it.  This makes for the opposite situation from the early Mitsubishi design in which tangs compelled the diaphragm to rotate with the cap; in the Dodge arrangement, the hard plastic allows the cap to slide easily over the diaphragm, which remains motionless on the reservoir.

Either the Mitsubishi fix or the Dodge fix works fine.  If either is available, the Dodge version is probably preferable because the cap turning against the hard plastic rim makes it easier to unscrew the cap after it's been closed for a while.  Note, though, that both the Mitsubishi fix and the Dodge fix require the tangs within the cap to be removed.