The proportioning valve is just a spring-loaded restriction in the line to the rear brakes that is normally open to fluid flow. There's no need to bleed it separately. Gravity-bleeding is normally sufficient but when the master cylinder is removed, getting the air out of the lines gets more involved. If you're pedal-bleeding with a helper, he may be pushing too hard causing an air bubble to compress and get stuck in a high spot. Instead of the "pump it up / push it down" method where you open the bleeder screw after fluid pressure is built up, try holding your fingertip lightly over an opened bleeder screw, then have the helper slowly and repeatedly stroke the pedal until clear fluid comes out with no bubbles. Your finger creates a one-way check valve so no air gets drawn back in.
If you have all the bleeder screws open at once, fluid will flow from the easiest route and the air won't come out of the hardest route. Do just one at a time. If that still doesn't help, you may want to try a vacuum bleeder. The best ones use compressed air to create a venturi effect. The vacuum will expand any air bubbles and make them easier to remove. There are hand-pump units too. The drawback to this method is you will almost always have air sneaking in past the bleeder screw's threads and into the clear discharge line, so it looks like more air is still coming out. You'll waste a lot of new brake fluid if you aren't aware of that.
There are also bleeder balls that can pressurize the system but they just speed up the pedal-bleeding method and make it a one-man job. The advantage is they have pressure-limiting valves that prevent the pressure from going over 15 psi. Above that air can be absorbed into the fluid making it impossible to bleed out.
Did you bench-bleed the new master cylinder before you installed it?
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Wednesday, January 2nd, 2013 AT 12:02 AM