In a nutshell, why wouldn't I want to replace the copper crush washers on the break lines with rubber washers or gaskets?

Subquestion: Is it necessary to replace the copper washers *every* time I loosen or remove the banjo bolt at the master cylinder?

I was told that copper washers compress and "flow" into surface irregularities and provide a tight seal, a must for brake and fuel connections.
Plain rubber washers, if squashed, break down after a while and cannot contain the high pressures in a brake line. Also, some brake fluids eat away plain rubber washers.
Recommend: OEM, teflon, stainles stell crush, and copper washers. Don't gamble with brake line connections. A few pennies saved can cost you your life.

Because your brakes won't work. Copper washers are designed to seal up to 2000 lbs of hydraulic pressure, rubber washers might hold 200 if you're lucky. The copper is actually a gasket, not a washer, it only looks like a washer. It is made of "dead soft" copper which has been anealed to be as soft as copper gets. You can mark it easy with your fingernail. You can re-use these coper gaskets as many times as will seal. I replace these only when I see damage to them or, if they have any kind of leak.

Please, don't mess around with the design of your brake system unless you are comfortible with hydraulic engineering. If a modification fails it will happen when you most wish it wouldn't have, under hard braking. If something fails then you die, somebody else dies, or at least all involved have a really really bad day. We're not ready to loose you around here yet............. B)

Best practice is to replace the copper crush gaskets every time, but I confess that I have reused mine a time or two in the past to no ill effect.

Of course, gambling with your brakes is gambling with your life, so do be careful.

--Bryan

Copper washers and rubber O-rings are not interchangeable in this application. Rubber o-rings only contain high pressure when they are seated in machined grooves. Copper washers have sufficient cross sectional strength to seal the 1500psi or so that your brakes will produce. A rubber oring in this situation will at first crush down to nothingness between the banjo bolt and caliper body, then blow out upon the first brake application.