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 . 

Are your reservoirs new? I had the same issue you’ve experienced and the solution was aftermarket reservoirs.

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?

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 

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.  

 

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 

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.

 

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.  

 

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.

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 

Your picture is the warning switch right?

Anyway, looks like fun.

How did you decide on what Orings to order?

On 1/18/2023 at 7:46 PM, Zed Head said:

Your picture is the warning switch right?

Anyway, looks like fun.

You are correct - I should use the proper name .