dklawson said:
Yes, there was a time when you had to be VERY careful about which fluid you used. The fluid compositions didn't change, the brake rubber did.
I have a not so humble opinion about this.
Everyone seems to forget the amount of NOS parts in the form of rebuild kits still floating around. And don't forget those aftermarket pacific rim parts. They pop up every few months and people snap them up on Ebay, or on sale at their favorite supplier.
It is highly likely some of these kits contain seals that are still natural rubber, or contain natural rubber in the seals, or just one seal in the kit contains natural rubber. Also if the kits are made in South Asia or Indoneisia, you have no idea what the seals are really made of.
US ester formulations of DOT 3 swells and softens natural rubber seals and causes them to fail rapidly. That is probably why your MC started leaking, or, you have a pit in the bore of the MC at the low spot where the seal rests in the bore which is also typical.
So in your LBC, unless you are 1000% sure all of the rubber seals in your brake and clutch system are new synthetic rubbers containing NO NATURAL RUBBER, then you should use ONLY Castrol Crimson, Sterling or LMA fluids, Dot 5, or a brake fluid that specifically states it is safe for natural rubber seals.
Can you flush all the contaminated water droplets out of your system so they will not sit against the seal and cause a corrosion pit? No. Why? Because that water droplet is in the low spot in the bottom of the master or slave cylinder and the exit and bleed hole is in the side or the top of the cylinder. Water sinks in petroleum products.
Disassembly or bench bleeding are the only ways to get the crud and water out.