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The brake Master Cylinder

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This series is divided into seven sections:
Part 1 - Why it goes bad and what it feels like (and why your pedal goes to the floor)
Part 2 - Removal and replacement
Part 3 - How to bench-bleed the new Master Cylinder
Part 4 - What the Master Cylinder looks like inside
Part 5 - How the Master Cylinder works, and why the pushrod needs to be set correctly
Part 6 - How to check the pushrod freeplay (a link to the bottom of Part 2)
Part 7 - How to adjust the pushrod freeplay
Note: All pictures can be clicked on for LARGER versions!

PART 3: How to bench-bleed the new Master Cylinder

To begin with, let's dispel that common story that bench-bleeding is some sort of mysterious, difficult black art. It's not; it's very simple. And if you're a typical home mechanic without specialized equipment or a vacuum pump, bench-bleeding makes it a lot easier bleeding at the wheels after installation, sometimes even making wheel bleeding unnecessary.

Your new Master Cylinder is full of air. When it's made, brake fluid is used as an assembly lubricant, but otherwise no fluid is added to it. The MC is complex inside, with lots of nooks and crannies among all the seals and springs, and it tends to trap air inside.
This is why we need to bench-bleed.

It is quite a lot easier to remove the air on the bench than it is to remove it once the lines are hooked up. And there's no point in feeding that air all the way through  the lines, only to be ejected at the wheels anyway.


A preamble...

The rebuilds available to me all came with the requirement that you reuse the rear rubber seal and the reservoir and cap. If this is the case for you, clean up your reservoir and cap carefully, and install it onto the new MC the same way it was on the old. Make sure you orient the screw clamp so you can get at it once the MC is reinstalled! (Just in case). Then put the rubber seal in place at the rear where the plunger is..

Remember: CLEANLINESS IS ESSENTIAL. I washed my hands several times during my work, just to get the grease off that would attract grit. I used spray brake cleaner, and went through a lot of it, making sure absolutely every trace of grit was cleaned off the reservoir, and cap, inside and out.

The cap is double-skinned, and I could not take it apart. When I cleaned the cap, I was careful not to spray too much brake cleaner on it, so it wouldn't get inside the double skin. Then I gave everything lots of time to dry thoroughly.


The actual bleeding procedure:

Clamp the MC into a vise as shown in the pics, making certain it's level!

My newly aftermarket-rebuilt MC came with a bleeding kit consisting of two plastic nuts and a rubber hose. If yours doesn't come with one, you don't really need it. By the way, the hoses came molded in one piece, to be torn apart like a Cheese-String. Tip: Tear them apart, but leave the free ends connected by an inch or so. It makes things easier later on.

Get an old Tupperware container to catch the fluid as it's ejected from the ports.

Update, one week later: I came across a post in another Usenet group which said that the tubes ought to be
looped back into the reservoir instead of into a Tupperware. I never thought of doing that. Ordinarily you wouldn't loop the fluid back through again, but in this case that may be more convenient than using a Tupperware bowl. And the fluid ought to remain clean as it cycles through.
Bleeding kit    Bleeding kit installed



I had problems with this bleeding kit, as the plastic nuts were a very tight fit, and I had trouble getting the threads to "bite". One went on OK, but the other one stripped and did not seal thereafter. (More on this below...)

Just a test here (photo below)  to see what it's like pushing in the plunger. I'm using a rod from an old set of vinyl window blinds. A wooden dowel, or anything non-metallic will do as a pushrod for this purpose.

The instruction booklet specified pushing in the plunger an inch with each bleed stroke. This is a MINIMUM. You can push the plunger in until it stops if you like, which is actually better than a bare one inch of travel. It takes a lot of force to move that plunger. Moving the plunger at least an inch ensures that both circuits will be compressed. For why that should be, find out How the Master Cylinder Works...

Ready to push in the plunger   Plunger pushed in one inch


I had to use both hands to bleed, so I have no pics of the actual procedure. Basically, it's this:
1) Fill reservoir with fresh fluid
2) Push in plunger and hold it
3) Pinch hoses shut with your fingers
4) SLOWLY release plunger (new fluid will get sucked in as you release)
5) Unpinch hoses
6) Repeat from #2, filling reservoir as needed (if you're draining into a Tupperware. If not, no need to top up).

There will be lots of air at first, but eventually you'll see fluid start to come out of the hoses. You'll also see bubbles in the reservoir. That's OK and is just some air coming through the ports. Another mistake by me: I left the screen in. Leave it out. The bubbles can escape better that way. Getting rid of all the air shouldn't take more than 3 minutes.

As you continue to bleed, the bubbles will lessen and eventually disappear. Looks like chicken soup, doesn't it?

Bubbles from return port


You can unclamp the MC from the vise once or twice and give it a few taps with a nylon mallet before reclamping. This will help shock any stubborn bubbles free.

When the bubbles disappear, you're done bench bleeding. Put back the little plastic caps that originally sealed the ports. This will keep fluid from leaking out. Put the cap on and walk out to the car. That's all there is to it.

And if you accidentally get air sucked back inside because you forgot to keep the ports closed off as you released the plunger (and didn't loop the lines back into the fluid), no problem. Just keep going. The air will still get ejected.

Fully bled, ready to install


Problems, problems!
As I said earlier, I had trouble with the bleed kit. Since one of the plastic nuts had stripped, air kept leaking past the threads back into the MC bore, causing foam the next time I pressed the plunger:

Foam from stripped threads on plastic nut

What could I do? I didn't feel like wasting time going to Canadian Tire to get metal lines and nuts, and after some thinking, I discovered I didn't really need the kit at all. I pulled it off (carefully picking out the shards of plastic from the stripped nut) and simply used my index and middle fingers (as in the "Peace" sign) to seal the ports when I let go of the plunger. It made a bit of a mess on the piece of 2x10 lumber my vise is mounted to, but otherwise worked very well.

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