Bleeding rear brakes

[Captain Obvious]

9 hours ago, Richard McDonel said:

 I think - not totally sure - I'm the wiser for reading your comments.

Perfect. That means we're doing it right.  

So to your final question about vacuum bleeding vs. pressure bleeding...

Again, I'm no expert on the topic, but I would suggest pressure bleeding over vacuum bleeding for a couple reasons.

First, I believe you can generate a whooooole lot higher pressure differential with the master cylinder. I didn't research it, but I would expect that when you push the pedal hard, you can generate hundred(s?) of PSI in the lines. But if you're drawing a vacuum, the max vacuum you can achieve is less than one atmosphere (less than 15 psi). So for pressure bleeding, your pushing fluid through the lines with hundred(s) or PSI, but with vacuum bleeding, your pulling fluid through the lines with less than 15. Seems you would be much more likely to generate that bubble-free "slug" of fast moving fluid if you're using a higher pressure differential.

Second, the seals used in the system are designed to keep pressure IN, not keep pressure OUT. All the seals are angled in such a way as to designate which side is the high pressure side and which is the low pressure side. The seals are all designed to expand outward and provide more sealing force when the pressure behind them increases.

I've run the exact same scenario that Zed Head did (bleeding a clutch slave cylinder) and when I released the slave plunger, it pulled air past the seal back into the fluid side. So either he got lucky, or I got unlucky, but what worked for him did not work for me. I had to pressure bleed because the vacuum generated when I released the slave plunger allowed air in faster than it would pull fresh fluid from the clutch master.

[Zed Head]

16 hours ago, Captain Obvious said:

either he got lucky, or I got unlucky, but what worked for him did not work for me.

Your points are all good ones.  The pulling of air through the seal would depend on the balance between the rate of pull (the spring force of the spring behind the piston) and the viscosity and availability of the fluid through the tube from the reservoir.  And the tightness of the seal on the piston.  So, lots of other variables besides luck.  But it's something to be aware of.

I think that the same things apply to vacuum versus pressure for brakes.  If you pump your vacuum chamber way down to close to true vacuum you're more likely to suck air past the seal I'd guess.  

And, applying all of this to the "corner" theory, you can see that a slow fluid flow to avoid sucking air could lead to leaving corner bubbles behind.  

In sum, considering all, for me, a shovel handle against the seat back remains my preferred method.

[Wally]

can i tag onto this thread... was bleeding all my front and back and read right looks bad. You can see in image about where fluid is coming out. i assume just replace the wheel cylinder. Is it that difficult to do?

[Yarb]

Easy to do but recommended you change both sides. Pressure differential sometimes will blow the other side. Check your shoes to see if they are wet.

[Zed Head]

It's not too difficult.  A screw or two on the backside, some brake line fittings.  If you're handy I think it might be possible to do without removing the shoes.  Shoe installation is more difficult, with all of the springs and clips.

 

Edited October 18, 2021 by Zed Head

[Richard McDonel]

YARB is right.  Change them both.  For a start if you haven't done the the job before, the first one will take too much time.  After that, the second one will take half the time.  Also, if one cylinder has failed, there is no reason to expect  its twin has much, if any life left.  

[w3wilkes]

Don't know which wheel cylinders you have? If you have the early wheel cylinders ($290.00) you can replace them with the later model ($50.00) with a little rebending of the hard line on the back plate.

Early / Late Wheel Cylinders? - Wheels & Brakes - The Classic Zcar Club (classiczcars.com)

Motorsport! Drums & Wheel Cylinders - The Z Store, Nissan-Datsun 240Z-260Z-280Z-280ZX-300ZX(Z31/Z32)-350Z-370Z Parts

 

[Wally]

not sure. i did see the $59ish dollar ones. It says on listing they will "fit" so i will likely buy a set of them. The oem's are $320 which is ridiculous

[siteunseen]

I found switching the short hardlines from passenger's side to driver's side & vise versa lessened the bending required. When you get into it give that a look. May be helpful? Was for my car.