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Master cylinder anomaly


madkaw

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1972 with all new brakes . Master cylinder is a NABCO , or whatever is being sold by Nissan - but made in Japan .

I am losing fluid but in a strange way . After a long period of driving I check the MC and the front reservoir is low  while the rear is full to the top . I have brake fluid oozing out somewhere from the rear reservoir . Not sure if it is somehow getting pressurized and coming out the bottom or what . I’ll have wetness around the band clamp and around the MC body . The band clamps are tight .

This is driving me nuts . I’ve never read of this situation . 
 

The front brakes are upgrade with the S12 calipers . 

8CEAE020-D413-4FFD-B9AB-DAC51A10B88D.jpeg

751CCC0B-3E50-4EA1-9F69-B674F4649208.jpeg

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You mention it is wet under the reservoir. It is possible that there is a small bit of something is caught between the boss on the master cylinder and the inside of the reservoir where it is clamped, or a burr or other imperfection in either part, resulting in a small leak. Brake fluid has a fairly low viscosity, and atmospheric pressure is enough to push it through any orifice. 
 

Also, you have inspected the entire system for leaks, correct?

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1 hour ago, Racer X said:

You mention it is wet under the reservoir. It is possible that there is a small bit of something is caught between the boss on the master cylinder and the inside of the reservoir where it is clamped, or a burr or other imperfection in either part, resulting in a small leak. Brake fluid has a fairly low viscosity, and atmospheric pressure is enough to push it through any orifice. 
 

Also, you have inspected the entire system for leaks, correct?

There is a transfer of fluid happening between reservoirs so it is overfilling - that’s the issue 

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Seems like the front seal is installed backward, or one of the middle seals.  It's pushing fluid from the front of the cylinder to the back on the return stroke.

Easier to imagine with a picture.  

image.png

 

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Well I guess I have my lines crossed . I replace the originals with SS - but the replacement lines were routed the same way as the originals . I contacted my Datsun part expert and shared a pic that helps . The F&R lettering on the BMC didn’t seem to correlate with the plumbing of the proportioning valve .

Still not sure that would cause the transfer of fluids though 

1C2536F9-DCFC-465C-98B8-6EEBD1CF24DD.png

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I had another thought about the brake check warning lamp switch.  It could allow a transfer but I don't know why it would be one way.

My first post was on target I think.  The seals push one way and let fluid slip past the other way.  Easy to check.  Good luck.

 

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16 hours ago, madkaw said:

There is a transfer of fluid happening between reservoirs so it is overfilling - that’s the issue 

Sorry, I missed that in your description of the trouble.

15 hours ago, Zed Head said:

Seems like the front seal is installed backward, or one of the middle seals.  It's pushing fluid from the front of the cylinder to the back on the return stroke.

Easier to imagine with a picture.  

image.png

 

What he said.

I would either exchange it for another, or if that isn’t possible, then disassemble it, and if the seals aren’t oriented correctly, set them in the correct orientation and reassemble it.

As Zed mentions, the seals have a specific orientation, and the cylinder may be bypassing internally.

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Not a very good pic but you might be able to make out the busted O rings on the piston . I’ve ordered some O rings . I will “ rebuild “ the prop valve and reinstall the MC with the lines routed properly 

3512F3CA-5F13-4ED2-82C0-600202AB519A.jpeg

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15 hours ago, Patcon said:

How did you decide on what Orings to order?

That IS the issue . It seems that EPDM O-rings are what’s best for brake fluid . Then you need the exact size . I believe the seal need to be 6x10x2 

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1 hour ago, madkaw said:

That IS the issue . It seems that EPDM O-rings are what’s best for brake fluid . Then you need the exact size . I believe the seal need to be 6x10x2 

Are they round cross section, or square?

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The MC seals are all oriented the correct way. The Nissan unit matches up with the aftermarket fine - see pic .

So I reached out to Classic Tube since it seems their kit doesn’t have the correct line routing MC to warning switch correctly . They show 3 kits by year . The first kit supposedly goes from 1969- 6/72 . The second kit goes from 6/72 until the 280z?. 
 

Am I wrong that the earliest BMC had front and rear oriented differently and thus had the lines routed differently to the warning switch? If so - when did the MC change ? 
Classic tube wants to get this right also . 

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Here's the drawing from the 1972 FSM.  It shows the big reservoir up front and shows the lines through the warning switch.  No drawing number though.

image.png

This from the very same chapter.  Seems reasonable to guess that it's an old picture from 1971..

image.png

BUT, I opened up what is supposed to be the 1971 FSM and it shows this.

image.png

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6 minutes ago, Zed Head said:

I'm not sure it matters though for function.  It's just a bigger reservoir.

If the larger reservoir is up front then more than likely the front brakes worked off of that side. So the lines to the warning switch would have to run differently - correct . The early lines stayed separated where as the later the lines cross - because the warning switch stayed the same ?

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Okay- I got the word from my parts expert .

This probably old news or non-news but I’ve just ran into this and was having difficult time finding the correct info 

240z 69-8/71 used first series of BMC - E4602 plumbed F-R 

8/71 - 6/72 uses the E 8702 series BMC - it is plumbed R-F 

7/72-7/73 also uses the W8702 but the safety switch is moved so it’s plumbed a lot different .

If anyone has contrary info I’d be glad to hear. 
If not I am going to notify Classic Tube 

 

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On 1/20/2023 at 1:29 PM, Zed Head said:

I'm not sure it matters though for function.  It's just a bigger reservoir.

 

On 1/20/2023 at 1:38 PM, madkaw said:

If the larger reservoir is up front then more than likely the front brakes worked off of that side. So the lines to the warning switch would have to run differently - correct . The early lines stayed separated where as the later the lines cross - because the warning switch stayed the same ?

If what @Zed Head says is true then it seems like you could just swap the reservoirs and be done with it rather than rerouting the lines to the brake warning switch.

Edited by w3wilkes
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1 hour ago, w3wilkes said:

 

If what @Zed Head says is true then it seems like you could just swap the reservoirs and be done with it rather than rerouting the lines to the brake warning switch.

Whatever is done the F of the MC should go to the front brake lines- and R should go to the rear lines - regardless of the reservoir size . 

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36 minutes ago, madkaw said:

Whatever is done the F of the MC should go to the front brake lines- and R should go to the rear lines - regardless of the reservoir size . 

I don't know if I would dispute this but why do you think this is so?

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I must recant... Looking at the top cross section diagram with the BR212 in the bottom right corner (assuming relational distances depicted are correct) the distance from the plunger to the exit pot is greater on the firewall side chamber of the MC with a longer shaft allowing it to move greater amounts of fluid both on the input side and output side. With the difference in the shaft length sections for the two reservoirs they will move different amounts of fluid. From the diagram I can't tell if the shaft is a single piece or is it two pieces and the brake fluid between two pieces of shaft moves the further shaft from the firewall. If that makes any sense... @Captain Obvious

PS- I've never personally had one apart so I'm totally basing this speculation on the diagrams.

Edited by w3wilkes
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I don't have pics of the older style of master cylinder exploded, but this is what the later one (with the front circuit closest to the firewall) looks like:
P1020127.JPG

Here are a couple other threads with pics and significant discussion about master cylinder operation:
https://www.classiczcars.com/forums/topic/66276-brakes-dragging/?&page=2
https://www.classiczcars.com/forums/topic/66168-brake-master-cylinder-identification/

 

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