Classic mini coolant?

A 1991 cooling system should be drained, flushed and CLEANED.

Classic Mini Radiator

Empty the contents by loosening the hose clip at the bottom of the radiator and pulling the hose off.

(you will get wet/dirty)

rinse with water then refill and run the engine for just a few minutes, drain, tighten the hose clip and fill with antifreeze mix.

Run the engine then check the level as air can sometimes get trapped.

Don't remove the cap from a really hot engine as the water will boil and scald your hand.

The bonus you get with commercial antifreeze is it contains corrosion inhibitors, stops the metal parts of the cooling system rusting away. If you have a brown soup it is likely the inhibitors aren't working or antifreeze isn't added.

It is rust. Yes you must flush or at least fill and refill until it runs clean. Then use 50-50 coolant to water ratio. I've had it rust like that only with only water.

HI Ben

in your case i would given that a cracked cylinder block is the last thing any of us wants.

these days instead of it being a water and antifreeze solution it now comes in a prepared preparation which you buy already with the correct amount with a little extra in case of seepage.

so gone are the days of having to mix it yourself.