prevent brake fluid flow back into master cylinder

[PerfectGrade]

Like others say in the thread, what the service manual is saying this for is in order to prevent debris within the system from damaging the master cylinder, and possibly the ABS modulator components.

The best way to go about this is to just fully bleed whichever corners you're trying to work on. This way, you're flushing out most of the debris and contaminants in the system which would risk damage to the master cylinder and ABS modulator. If you feel bleeding alone is not enough, the extra step (which has earlier been mentioned) is to open the bleeder (hopefully with a drain tube still attached) as you force the piston back into the caliper. If you chose to do this 'extra' step, you may want to just bleed it a little to make sure no air made its way past the bleeder screw.

I personally never replace pads on a caliper without bleeding it before hand. It ensures I have the system clean before forcing the piston back in, and the piston does take slightly less effort to get moving.

Hope this helps.

 

[ikarus1]

1. Buy a vaccuum 1-man brake bleeder kit
2. vacuum disgusting fluid out of master cylinder
3. bleed all ancient disgusting fluid out of all 4 corners
4.install new friction linings.
5. drink cold beer

that's how I make sure my cars stop as well as they go.
The vac tool I use is the one that Harbor Freight sells for around $25 that uses venturi vacuum, and a MC reservoir bottle.
You can use a mityvac type vac tool, but they give me hand cramps lol. You can also use the pressurized motive type, but then you have to have all kinds of master cylinder cap adapters.