Very much doubt that will do anything except spoil a good pen, did you actually make it? or did it sound cool to have such an exotic mix?....no disrespect but looking through this thread a lot of people seem to be doing just that............
Brake fluid thickens stuff up because its an oil whilst the thinners will make it thin again....kinda defeats the object....
Etch is usually a water based acid and seeing as oil and water dont really mix i wouldnt put brake fluid in there....
Got to remember your chemistry when you are mixing stuff together...
WATER BASED.. etch, emulsion (bucket) paint, ect
SOLVENTS - made with a oil based thinner (these are usually the smelly ones!) ...Permanant inks, pilot, shoe dye, thin paint, paint stripper, petrol, nail varnish remover ect...
OILS - brake fluid, hydrolic fluid (very simular to brake fluids), gloss bucket paint, shoe polish ect
You can mix oils and solvents quite easily to thicken up or thin down an ink but waterbased substances are a definate no no....