Yes, a diy bfc is full of possible "pifalls", sheared of nipples etc etc etc ...
Yes, suck out master cylinder resovir "old fluid" else the crap ends up in the haudraulic system, which is where it came from in the first place.
Yes, fluid needs changing even the modern less hygroscopic stuff, which can still have internal corrosion problems.
Make sure yo have the correct fluid and ideally change every 2 years. Merc even say seals need changing at that time.
Bleed nipples, clean out nipple central holes with a small drill ( do no, DO NOT break the drill in the hole!!!!! )_ being aware that the hole has a smaller diameter further in. Wire brush/clean the nipple exposed thread and penetrent penetrent penetrent. us esome windscreen washer tubing pointing upwards and fill with penetrent, and leave it to do its thing for as long as possible. The longer the better.
Whilst you are there, wire brush the brake pipe union and penetrent that as well. If you need to renew the wheel cylinder, the brake pipes can seized/corrode into the union, twisting the pipe if the union needs to come out, meaning a new brake pipe etc.
When undoing the nipple, no jerking just steady torque and hopefully it will undo with no issues.
Initally I tend to let gravity help with initial bleeding, with the master cylinder res cap off. When discoloured fluid changes to new colour i shut that nipple off and do the rest in turn.
When all gravity bead, then my assistant is in charge of the brake pedal followning my commands.
1. pump up a few times and then hold pressure, she complies and says so.
2. i undo nipple and watch fluid come out via the clear tube andinto the jam jar. I do the nipple back up. And say up. She complies and says so.
repeat 1 untill satisfied no more old fluid and no air, then move on to the next nipple.
Its usual to bleed the nipple furthest from the master cylinder firt etc, keeping a very keen eye on the fluid level.
Some master cylinder res feed the haudraulic clutch as well, so bear that in mind.
! to 2 litres should be more than enough for a flush and bleed, usually. ALWAYS use new fluid from a sealed container and dispose of old fluid properly.
Check your car manual for any "reset" needed for the cars system.
Remember that the master cylinder is a pump, with the pump being able to pump fluid and not drag in air, so a proper procedure is needed else spend the day and gallons of fluid.....
A slightly pressurised master cylinder res is a single person tool as the res is automatically topped up when the tools is in use.
Its pointless replacing the fluid if the brake system has faults like stuck caliper pistons or bad hoses and leaky wheel cylinders etc. etc. etc.....
so, suck out res old fluid and refill res, pump up command to assistant with her reply, down command with her reply, open nipple and watch fluid flow then close nipple, up command with her reply, wait a few seconds and then repeat. When new fluid an no air, that nipple is blead.
Modern cars have 2 haudraulic circuits. When bleeding only one circuit is bleed, so pedal will only go about half way down, so only go 1/4 way down so that circuit is only at half stoke........
Take your time, dont break anything and dont let the res run out of fluid, ever!