Welcome to this forum!
You should introduce yourself, stating wrenching capabilities, experience and resources…answers to inexperienced new vintage Volvo owners will be different from those to more seasoned hands-on Volvo wrenchers, who do know the difference between a carb dashpot and a marijuana cigarette on the dashboard…
That said, I will answer your questions, and my answers come from 30+ years of firsthand experience with several cars…and I’m a total advocate for DOT5, moreso apparently than JIH…
If you are considering changing to DOT5 silicon, lines, flexlines, and components do not have to be changed, but a good flush is necessary and required. DOT5 does not play well in direct contact with DOT4 (and this includes the non-silicon based Red Rubber Grease included with Girling rebuild kits and used to lube internals). I have experienced the RRG turning into a semi-crystaline, semi-gooey munge, which effectively seized a wheel cylinder, when a brake system originally built and filled with DOT4, was subsequently changed to DOT5 with a flush consisting of nothing more than a full reservoir of the DOT5 through each corner at bleed time).
Stainless Flexlines are internally actually Teflon lines with the SS braiding acting as an external protection. SS flexlines are better from the standpoint of no long-term internal swelling/occlusion due to rubber reaction/swelling with the fuild, and also no short-term swelling due to internal line pressure (so give a better “brakefeel” when threshold/performance braking, although this may not interest you so much!). I will only install Teflon/SS lines for the former advantage, less-so for the later…I like doing things once and right, and dislike redoing things for no good reason!
My most recent experience with a DOT5 converted chassis (full SS line and component [built with silicon grease] replacement in the mid ’80s), was taking it out of long-term (dry) storage, and having the brakes work every bit as well as the day we put it in there!!…no leaks no runs, no drips, no errors!!! I’d like to see any DOT4 equipped car do that…or even still have brake fluid left in the brake (or clutch) reservoir after that amount of time!
JIH; “DOT 5 became kind of a rage back in the 70’s as word spread the military was using it. The military later on abandoned it. ” That’s kind of a loaded, unsubstantiated statement designed to dissuade someone from considering it…silicon brake fluid would have never gotten a DOT designation if it had not undergone rigorous testing to confirm that it was least suitable, so any discussions we vintage car operators engage in must be geared to pros and cons and long-term and short-term performance aspects…that said, I wasn’t aware the military has discontinued its use (I’d like to see further info on this, and why they “abandoned it”). …back on the Brickboard Forum, the very experienced, highly regarded (in the Volvo community), and former serviceman George Downs (now departed), also a big advocate for DOT5, told first-hand stories of his military service in Panama (where the humidity is 300% on a “dry day” without rain!) and of the mandatory and highly successful use of non-moisture absorbing DOT5 in the vehicles operated in the jungles there…I don’t expect they called for its use because it didn’t attack the olive drab paint on the vehicles! I do expect military vehicles to rarely get their brake fluid to the high temps where DOT5 IS compressible, and that makes it totally unsuited for racecars (and probably bikes too, but you sound like you’re better placed to know about that)…
Cheers
Edit: I hadn’t seen 142Guy’s response, but I agree with him…modern seals are made of synthetic EDPM rubber compatible with both DOT4 and 5. There is no campatibility issue that I’m aware of. See also: SW-EM Girling 3 Piston Caliper Rebuild