I'm replacing the rubber kit in the single circuit master cylinder, and the kit package that I bought from Mini Spares Centre came with a part that I don't recognise. I consulted my haynes workshop manual and it says that a non-return valve (which is what I believe this part is) is placed on the large end of the spring, yet it is not present in the exploded view of the master cylinder.
When I pulled apart the master cylinder I couldn't find one. I found that strange as I had the master cylinder refabricated with a stainless steel insert a while ago.

How important is this part?
and what exactly does it do?

The non return valve is a part on drum braked minis so that they hold some pressure in the lines the non return valve is binned if you have disk brakes.

As a result some master cylinders have return valve seal thingys and others don't I guess...

EDIT: Oh Wow I'm now a 998 <--

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Roads need more corners
A Deluxe(CG13DE), 2 Clubbies(998 and 1275) and 2 Morris 1100Ss

The valve is called a residual pressure valve and its purpose is to maintain a low pressure in the brake lines when the brakes are off to ensure the seals in brake wheel cylinders on drum braked cars are slightly expanded. This is supposed to reduce the possibility of air entry into the wheel cylinders. It is still necessary if you have discs at the front, drums at the rear and a single circuit brake system. Its not necessary for the disc caliper seals as they don't rely on residual pressure to prevent air entering but they are still needed for the rear drums. If you have a split system with front discs and rear drums you only need the valve in the rear brake circuit.
Don't believe everything that you read in Haynes manuals as the few I've seen have contained many errors and not only for Minis but for other makes as well.
Others may have different opinions but BMC/Lockheed wouldn't have fitted them if they weren't necessary.

Regards
RonR