On doing brake line bleeding, I have had the experience of breaking off the bleeder valve. Is there any reason not to reverse bleed the fluid by pushing in the brake cylinders as you do when you are replacing the brake pads?
You can’t do it that way. The same old fluid would remain in the brake hose and brake line. If the broken off bleed screw doesn’t lose brake fluid, you could disconnect the brake hose at the caliper. Pry the brake caliper piston in to push the the brake fluid out of the piston bore. Depress the brake pedal until fresh brake fluid comes out the end of the brake hose. Reattach the brake hose to the caliper while the brake fluid is still being slowly pushed out the hose (by operation of the brake pedal).
Ideally, one drills out the broken bleed screw (if possible), or, replaces the brake caliper. Ideally.
You need to get the old fluid out of the lines. My understanding is the water that gets into the system is heavier than the brake fluid and will sink down to the wheels cylinders.
So, no you need to push the old fluid and contaminates out the “normal” way.
Regular bleeding keeps the system clean. I have had brakes with over 17 years on them with no corrosion. Then again, this is my track car gets bleed, as a minimum, after every weekend at the track. Brake heat will boil the water in the lines and cylinder causing a spongy pedal, not just corrosion.
Brake fluid will absorb moisture out of the air.
I think that used to be more of a problem than it is now. I haven’t broken one off since my '68 Dart. Try PB Blaster it well before trying to open it and take it very easy easing it open.
Don’t get you “sink down to the wheel cylinders comment”
You do correctly state the absorb the moisture property, (several words to describe this condition I believe all are correct)
I believe the water and the fluid mix (homogeneous) Don’t know if they will seperate if left unagitated (like in a jar left standing)
It 's those small diameter bleeders that seem to break off,bummer when that happens, never heard the process of pushing the old fluid back into the system called “reverse bleeding” I have always called it incorrect.
QOUTE “Don’t get you “sink down to the wheel cylinders comment” …
I believe the water and the fluid mix (homogeneous) Don’t know if they will seperate if left unagitated (like in a jar left standing)”
Yeah you could be right…I tried to remember where I picked up that bit of wisdom. I think it was in an Auto class. You got me thinking. In my limited, non professional experience, there is more corrosion happening at the wheel. The brake fluid doesn’t flow in the system but transmits pressure, so maybe the water does concentrate down stream, but not separate. Any event the stuff that comes out of the bleed screw looks nastier than the reservoir. That is the point get that nasty stuff out…do not push it back into the system.
I believe the water and the fluid mix (homogeneous) Don’t know if they will seperate if left unagitated (like in a jar left standing)
brake fluid is hygroscopic meaning it absorbs water. The 2 will not seperate.
Thanks,I new they were homogeneous (as stated) but I didn’t know if once mixed they stay mixed. Do you know there are several other words (I can think of 2) describing a substances “afinity” or disposition to attract water? I have seen all three used on the Forum. My text book likes to use the word “hydroscopic” notice the difference in the word you have used?
OK, so I did some reading up on brake fluid specs…appears I am all wet when it comes to water settling in the lines. I even went so far as to pour some water into brake fluid in a glass. (I know, I need to get out more.) The water did just sit on top of the fluid at first, like well, oil and water 
I put about 25% water to brake fluid and mixed it up. After a day there was no visible separation or “stratification”.
We all agree on the answer to roblarjo’s question: Bleed in the regular direction.
What I did find interesting from my reading is the rubber brake lines apparently allow atmospheric humidity to penetrate into the system
You got it right. I think folks think the water collects in the calipers/brake cylinders because the rust and dirt does collect down there, making the first fluid that comes out during bleeding very nasty looking. But the water will be evenly distributed throughout the system. Therefore, I would replace all the old fluid in the reservoir before I start bleeding, that way you’re not diluting all the new fluid with old, water-contaminated fluid.
Thanks for all the comments. The reason for the question was that I was replacing a master cylinder and thought that it would be much easier to “reverse” bleed as any air would be in the lines right at the master cylinder and just a bit of a reverse flow should get the air out.
Because of all the rust, dirt, etc in the calipers, I wouldn’t want to force that stuff back up through the system, especially if you have ABS.
This happens with rubber AC hoses also. This is how you get moisture in the system even if it isn’t opened,newer hoses are better preventing this.