How much should this cost?

And they work perfectly well with water or piss so long as the brakes are not applied long and hard enough to get the fluid up to boiling point. The MOT test won't detect if either is present.

How long the brakes will continue to function with water or piss in them is THE issue. Rest assured that it won't be very long.

The reason for changing it, in something which you are going to keep, is that replacing corroded slave cylinders gets expensive. With disk brakes, unless you take caravans up and down alps, the average road driver is unlikely ever to experience brake fade IMHO.

Ohh, where? :)

Caerphilly.

Only about 200 miles away, then. :)

Obviously, new brake fluid is put in when changing the brake pads or discs? At least I thought that would be the case...

Its the sensible option but not always done

Not at all. Brake fluid is generally changed on a time basis. The length of time pads last depends on lots of things.

My tuppence worth

I was in a queue of traffic going down Richmond Hill in S.W London (never been in the Alps, or towed a caravan) Brake constantly applied as its steep and we were moving at walking pace. After a few minutes the brakes started to feel odd .The free play in the pedal travel disappeared and I had to apply a lot of force to get any braking at all.My first thought was that I had lost the servo. Within a few seconds I was standing on the pedal(rock hard,no free play)but not getting enough braking to stop my car moving at 5mph

I had to ram it into first gear, turn off the ignition, yank on the hand brake and steer it into the grass banking to get it to stop In faster moving traffic I (and other road users)could have been in serious trouble.I never quibble now over a bit of brake fluid.

snip

.T

just discovered these symptoms are for brake fade due to overheating not for vapour lock apologies

steve