principle of jump leads

You can refer to anything you like, I'll refer to the AA

Any fluid in a switch can cause it to overheat & catch fire, you're not going to set your brake fluid on fire by jump starting your car.

Ah, so you've gone from 'don't put inflammable liquid in your brake system' to 'oh, any fluid will burn'...so you have learned something.. brake fluid IS inflammable..would you now connect the -ve to the master cylinder - please video the result for us.

You sidestepped the 'petrol' issue - you've got more chance of igniting petrol (or indeed brake fluid) than blowing up your batteries hydrogen content..

I point you to:

note - removing AFTER you have taken off the cables..and any spark, so as likely to be cause by a spark from the ht system..

Ian Dalziel gurgled happily, sounding much like they were saying:

Fine. You're not a matter. You're a don't matter.

Connect positive to positive.

Negative on donor car first then negative on flat battery. The idea being here that the donor car battery may have some hydrogen floating around (unlikely) while the flat one won't, so any spark won't matter.

Start the donor car. Run for a few minutes to charge the flat one. Then attempt a start.

Well I've no idea why you'd mistake the brake master cylinder for the engine block, although in your case I wouldn't be surprised & if you do the spark still won't set the brake fluid on fire

The difference being the petrols inside the pipework, the hydrogen isn't. If you really can't tell the difference between the flashpoint of petrol & brake fluid then you're very confused

The HT system makes sparks inside the engine, not directly above the battery. If you've been pouring petrol over your engine then the starter motor makes more sparks than anything else.

Plus the explanation why collisions involving Vauxhall Cavaliers frequently resulted in fires. The brake fluid reservoir was not very securely fitted to the bulkheads, and tended to burst free.

I noted that info because I used to drive one...

The issue here is flash point temperature. A single spark or application of a match won't ignite a pool of flammable liquid if it is below it's flashpoint - the vapour pressure of the fuel is too low to form a flammable atmosphere. This is the case with diesel which has a flashpoint over

But surprisingly petrol can also be pissing all over an engine and not always go bang. Back in the 70's , Austin Maxi with a fuel pipe directly above the inlet manifold, which was also right over the exhaust manifold. 70mph down the M1, hot engine, smelt something, pulled over and petrol was boiling on the pockets of the inlet manifold and cylinder head casting. Cause was a split fuel pipe under a clip on a support bracket - iirc the fuel pipe ran right over the cam cover. No fire, no big bang, just evaporating petrol :)