DIY: what can go wrong in the worst case scenario when changing oil, brake fluid or spark plugs, and what are the consequences?

A friend of mine does all of this work himself. He also repairs his tractors. He said oil changes, brake fluid changes, and spark plug replacements can be done by himself—but what's the worst that can go wrong, and which of these jobs are the least dangerous?

Brake fluid just below “Min” – top up yourself or take it to the workshop?

Hello, The brake light on my motorhome's speedometer has recently been permanently lit. The reason is the brake fluid, which is just below the minimum level. Can I simply top up the brake fluid, or should I go to the garage and have it done or have the entire brake system checked? What do you…