Bleeding rear brakes

[Zed Head]

8 hours ago, cgsheen1 said:

It enters the wheel cylinder NEAR the top...

Maybe that's a reason people have problems bleeding Z brakes.  You can imagine a pressure or vacuum bleeder that's moving fluid slowly leaving some air behind.  I put speed bleeders on the back mainly because I wanted to purge the lines of old fluid easily but it might have helped me avoid some problems also.

And, are those aftermarket cylinders?  Might not be representative of Nissan.  But, it does show angled drilling, maybe they just didn't get the angle right.  Might be some guy on an assembly line eyeballing it.

Thanks for the informative pictures. 

[Richard McDonel]

Captain,

OK, I think I follow what you are saying, but it seems to assume that the air is ahead of the fluid in the lines.  What about air that has fluid both ahead and behind it?  Air that got in because of a leak somewhere else - front brakes or proportioning valve for instance.  It seems to me that a guy is going to have to push and awful lot of fluid out before he gets to the bubble?  Does that make any sense?

 

ps, For the record I did not flunk Fluid Dynamics because I never took the course, so you're ahead of me there.  

[Patcon]

I  sometimes run ½ a quart through before I'm happy with how it looks. Mainly because a I want clear fluid coming out. No black. The bubbles normally go away pretty fast unless the line is really empty

[grannyknot]

5 hours ago, Richard McDonel said:

It seems to me that a guy is going to have to push and awful lot of fluid out before he gets to the bubble?  Does that make any sense?

But you have to remember that the inside diameter of the brake line is just 3mm so there is not much volume to begin with and when the pedal is pushed the fluid rushes through the line quickly taking air bubbles with it.  One possible problem with bleeding the brakes on our cars is that the farthest rear brake from the MC is not the P/S but the D/S, the D/S rear brake should always be bled first then the P/S or air bubbles could get hung up at the 3 way connector.

[Namerow]

Even if CO had completed that undergrad course in fluid dynamics, it wouldn't have taught him anything about the behaviour of bubbles entrained in fluid flowing through a pipe.  That's pretty specialized stuff.  It definitely wouldn't have taught him anything about dislodging a bubble trapped in the corner of a casting.

It's worth pointing out that applied science (engineering) is usually based on observing a behaviour first and then finding ways to apply the theory so that it generates a decent prediction. 

Grannyknot makes a valid point about the ratio of the bubble diameter vs the brake pipe diameter.  However, even if the brake pipe was 1/2" in diameter I suspect that the bubbles would stay small and would still be swept along with the flowing brake fluid.

This would be a great weekend project for CO:  Build a demonstration rig using clear tubing and some drilled-out clear acrylic 'castings' and then pump some fizzy brake fluid through it to see what happens at the high points and inside the casting voids. 

Somewhat off topic, but microbubble technology is a pretty hot topic these days in the field of medical treatments and drug delivery technology.

[Captain Obvious]

8 hours ago, Richard McDonel said:

What about air that has fluid both ahead and behind it?  Air that got in because of a leak somewhere else - front brakes or proportioning valve for instance.  It seems to me that a guy is going to have to push and awful lot of fluid out before he gets to the bubble?

I don't think it would be much different. That's what I meant about the fluid flowing fast coming out of the master cylinder. The reason it would flow so quickly through the lines is because of the small volume that actually exists inside the brake lines. Grannyknot's put it well above, but let's put some numbers on it...

If the ID of the lines is .118 inches (3mm), then the cross sectional area of said tube is .011 square inches.
That means in a ten foot run of brake line, there is only about 1.3 cubic inches of brake fluid.

I'm thinking that even if there is a bubble in the middle somewhere, it's just going to get pushed out ahead of the next slug of liquid when you're bleeding the system.

[Captain Obvious]

1 hour ago, Namerow said:

This would be a great weekend project for CO:  Build a demonstration rig using clear tubing and some drilled-out clear acrylic 'castings' and then pump some fizzy brake fluid through it to see what happens at the high points and inside the casting voids. 

Haha!!! I'll get right on that!

Or not.    

[Richard McDonel]

Gentlemen (I hope that's the appropriate salutation),

I really do appreciate your interest.  I think - not totally sure - I'm the wiser for reading your comments.  Here's a final question:  Is the use of a vacuum pump for bleeding any less effective than having someone pump the pedal?

Have a good weekend all - and a Happy Thanksgiving for my Canadian friends.

 

[Zed Head]

4 hours ago, Namerow said:

dislodging a bubble trapped in the corner of a casting.

This is the main problem with bleeding brakes.  cgsheen1's pictures show that it could still be a concern.  The "calipers on the wrong side" issue is an example of a large "corner" in the flow path of the fluid.  The rear wheel cylinder picture shows a small corner.  Really you're talking about dead zones that don't get swept by the fluid and are higher than the flow path of the fluid as it moves to the bleed port.  That's where the bubbles go.  Be the bubble.

If you had cgsheen1"s cylinders you might want to lift the side your bleeding waaay up in the air before starting.

Edited October 8, 2021 by Zed Head

[Zed Head]

The picture reminds me of another way to bleed cylinders - work the system in reverse, from the end.  I did that on the clutch slave cylinder.  You can open the bleed screw and push the piston backward with finger pressure.  That drives the fluid out of the bleed hole, then when you close the bleed screw, it pulls new fluid in from the reservoir.  You can also unbolt the cylinder so that it has the screw hole oriented upward.

This is in the same vein as pressure versus vacuum versus mechanical.

Edited October 8, 2021 by Zed Head

[cgsheen1]

I am the bubble.

I am the bubble.

I am the bubble...

Nope. Didn't help. I guess I'm just not Zen enough. 😝

[Patcon]

I have 2 different style vacuum bleeders but I still prefer pedal bleeding. I dont know if you've noticed but air can come in around the threads of the bleed screw.  That can't happen with pedal bleeding.  Fluid can come out but air can't come in