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Brake Master Cylinder

3 posts in this topic

220px-Master_cylinder_diagram.svg.pngI have to replace my master cylinder and I'm wondering how something  so simple can fail.   I've bled it forward and reverse and I just can't get it to pressure up, so i have a few questions:

1 When you adjust the brake pedal, is there supposed to be any pressure from the push rod on the plunger or a little free play, looks to me like you should have a little free play.

2 What's the best brake bleed method?  

(A)The old tried and true, 2 people required, method of pump the brake pedal until you get pressure, then open the bleed valve and reclose and over and over and over.

(B) Put pressure on the master cylinder reservoir and then bleed at each wheel cylinder, making sure you keep fluid in the reservoir.

(C) Put a vacuum on the wheel cylinder bleed screw and suck fluid out of the master cylinder.

(D) Drain or partially drain the master cylinder and pump fluid in at the wheel cylinder.

Are there any other methods that should be included here?

I tried B C, and D and still no couldn't build pressure at the pedal.  I disassembled it and reassembled it several time, and still, no pressure, all looks good, it just doesn't work, expect new master cylinder Monday afternoon.

Need to check for cracks in the master cylinder, the way mine was mounted, the banjo nut was in contact with the fire wall. On my 2015, I have the clutch and brake master cylinder on the front of the fire wall and there is a cut out for them to mount on and it should probably be a bit longer for clearance of the banjo nut. - I'll add a photo tomorrow -

Do you want to see a really bad place to have no brakes, copy this partial GPX and drag and drop onto Google Earth  -  https://drive.google.com/drive/folders/0ByQAhs0e-yF9NTdma1FNM3p2OUE?usp=sharing




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I'll presume you've removed most of the air from the master cylinder.  The usual process of that is to bench bleed the master cylinder before installation.  If you use method A, make sure your pedal pumper keeps their foot on the brake pedal and ride it down to the floor while you open the wheel cylinder bleeder valve.  Tell him not to take his foot off the pedal until you close the bleeder valve.  I make sure I tell my assistant that every time.  My assistant may not know how the bleeding process works.

Method B, in my opinion, is the best followed by method C which is good.  Method C uses a hand operated pump which is less expensive than the setup required for Method B.  I wouldn't use D - to easy to introduce air or contaminants.

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I have an air pump designed for a solder sucker that tested out as 10psi that I use to pressurize a Mason jar that I fill with brake fluid and then, using various means, pump the fluid through the system. I did this from master cylinder, then from the caliper and still really poor brake pedal, finally used some welding clamps and collapsed all the calipers to where the brakes were locked and started pumping fluid from the master cylinder reservoir to the calipers and while the fluid was flowing, also pumped the pedal several times, I did notice that when I pumped, I had air come out with the fluid, after that, I had good pedal, FINALLY!

Next, all the fluid that I recovered was filtered through coffee filters and reused in the system, or dripped or spilled, and now that I'm finished, saved for next time, along with the mason jar and tubing.


PS. I do agree that method D from above would tend leave any air in the system because the fluid is going in at a high point with no buffer like the reservoir so the air would just swirl around in the pressure chamber

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