Bleeding rear brakes

[Richard McDonel]

I have a 240Z.  The brakes work fine, but I can't understand the bleeding process.  Why is the bleeder screw on the rear drums at the bottom of the assembly?  Old Chevs and even back to  DKWs I've worked on always had the bleeder at the top, which makes sense, because air rises and we want to get it out of the system.  What am I missing here?

Thanks

[SteveJ]

I just make sure the bleed bottle is higher than the bleed screw. I haven't had any issues with bleeding the rear brakes.

[cgsheen1]

Is it though? Or does it just seem that way? Never-the-less, people have been bleeding these successfully for 50 years.

I use a slightly different bleed method than most. I bleed the master first (in car, using the bench bleed method), then the left front, right front, right rear, and left rear. It doesn't take more than 3 pedal pumps to bleed each corner this way. My bleed hose goes up - above each bleeder valve.

[Zed Head]

[Yarb]

@SteveJ Not sure I’ve heard this before about collection jar being above the bleeder. Is this necessary for the drum brakes in particular? Currently bleeding my new system with speed bleeders. Have a great pedal without booster assist. Fire the car up and pedal goes straight to the floor no resistance. I’ll start a new thread from here.

[SteveJ]

9 minutes ago, Yarb said:

@SteveJ Not sure I’ve heard this before about collection jar being above the bleeder. Is this necessary for the drum brakes in particular? Currently bleeding my new system with speed bleeders. Have a great pedal without booster assist. Fire the car up and pedal goes straight to the floor no resistance. I’ll start a new thread from here.

If the jar is above the bleed screw, the thought is that the hose between the bleed screw and jar is also above the bleed screw. If that hose is filled with fluid, you are less likely to pull air back in through the bleed screw.

[Richard McDonel]

Thanks for all your thoughts and suggestions folks.

What I'm thinking is that any air will naturally rise to the highest point available.  Therefore there is not going to be any air at the bleeder screw; it would have already risen to its highest available point, that being the hose where it clips to the top of the brake backing plate.  Regardless  of how high one holds the bleeder jar, if there is air in the brake hose at the top of the assembly, that air is not going to be sucked all the way down to the bleeder screw.  Isn't that why with the front discs the bleeder is at the top of the caliper?  

[Zed Head]

I think the key is where the exit hole is inside the cylinder.  Sometimes they drill at an angle and the position of the external threaded hole is misleading.

Anybody have a spare cylinder they can take apart and examine?

[cgsheen1]

Tomorrow I'll be replacing rear cylinders -  discovered that my left rear was leaking yesterday. Two new rear cylinders are being delivered to the shop in the morning... While I don't know if the bleeder comes off the top top, I think it's an optical illusion to believe it's on the bottom and there are cavities inside that could retain air during the bleed process. That goes against everything humans building cars have known about hydraulics for the past 100 years. 

[Captain Obvious]

Richard McDonel, I believe your asking "Why wouldn't air bubbles get trapped inside the hard line or rubber lines since they are higher than the bleeder screw on the wheel cylinder?"

I'm no fluid flow expert, but I'm thinking the fluid coming out of the master cylinder flows so fast that it pushes virtually a full slug of brake fluid liquid along inside the small diameter lines. 

The slug of fluid doesn't really want to separate and it just pushes all the air out ahead of "the wave", and even when you lift the pedal to get ready for another push, even if a bubble were to flow out of the wheel cylinder, it won't happen fast. You'll blow that bubble out on the next push.

Capillary action and vacuum lock and all that...

That's my non expert theory anyway.

Edited October 7, 2021 by Captain Obvious
little more detail

[cgsheen1]

So, here it is.

The bleeder is tapped and drilled at an angle. It enters the wheel cylinder NEAR the top...

The brake line feed is drilled perpendicular and enters at the center of the cylinder. 

I think Cap has it with his excellent Fluid Dynamics instruction.

[Captain Obvious]

Good pic.

As for the fluid dynamics... I had one class in fluid flow and thermodynamics*. I consider it the worst class I took in my entire educational career, and I got zero out of it. At the time I had absolutely no interest and (as an EE) could see absolutely no practical value to me. So it bounced off my brain and I managed to scrape through with a "C" using short term memory and a stiff grading curve (because pretty much everyone tanked the tests).

But now... Now that I've got a whole bunch of automotive related applications, I bet I would get a whole lot more out of it.

 

*Probably told this story before, but I'm gonna do it again anyway.