The brake and clutch master cylinder on most Minis in the USA and the single line version. During early production these masters included a metal reservoir for the fluid. These not only had a habit of terrible corrosion and look bad but you could also not tell how much fluid you had in them unless you unscrewed the top. In the latest design version (which is completely interchangeable with originals) the reservoir is made of a lifelong plastic that will not corrode and most importantly provides and easy way to simply see if you have enough fluid in them.