So after rebuilding my Z's braking system a few weeks ago, I have some questions about the engineering justifications for several of its features:

1. The front and rear braking systems are completely independent, yes? If so, then why the proportioning valve? Why not do all the proportioning through the master cylinder?

2. Is the function of the distribution block under the MC simply to split the lines from the one leaving the MC into the requisite number to feed pressure to each wheel? No valving in there?

3. What exactly does the sensor on the bottom of the distribution block sense? Low pressure? High pressure? Air in the system?

As information, my car has the early braking system with the proportioning valve back in the rear axle area. Thanks for any and all insight.

3. It's actually a switch coming from the block below the master. It completes the circuit to the red brake light in the speedo (tach? - one of them) when there is enough pressure imbalance to move the switch forward or backward.

1. I believe the 240z's proportioning valve is just a fixed orifice to reduce pressure to the rear brakes so that they do not lock before the front do.... pull your ebrake when cornering (preferably in a huge parking lot with snow) just to experience this effect. The proportioning valve could be incorporated in the master but it must be placed after the differential brake pressure switch that Zed Head mentions above(otherwise the pressure difference with cause the switch to falsely trigger).

2. Was answered by Zed Head. To add to it, the fluids to the back and front never mix. The differential brake pressure switch has a plunger between the front and back systems that gets pushed when one of the systems leaks and looses fluid and pressure. The plunger has seals that prevent mixing.

The "rear proportioning valve" on the early cars is essentially a hydraulic timer. It has a spring loaded cylinder that fills with fluid when you step on the brake pedal to delay the operation of the rear brake shoes. (giving time for the front brakes to engage first.) When you let off the brake pedal the spring pushes the fluid in the cylinder back towards the master cylinder to "reset" the time delay. By the way, these valves are NLA...

The later cars (starting in 73 or 74) have a true proportioning valve in the engine compartment.

Thanks for all the helpful responses, guys! That's great.

The knowledge of what causes the red brake light in the speedo to come on (imbalance between the front and rear systems, not necessarily low pressure system-wide) is very helpful. I'll bleed the system again and see what happens.

Does anyone know of a source for the electical contact sensor (the part that the electrical wire connects to) that is on top of the Assemby- Switch Brake Indicator?

I could only get the parts from old junk yard z's. Only the complete switch (part number 46100-U5200) is available on line for ~ $175 USD.

You may have to get a couple of used ones to refresh and rebuild into a good one as they are usually rusted and gunked-up inside.

Here is a great image I just found:

And if you need the rear proportioning valve for an early z just $20 right now on ebay: http://www.ebay.ca/itm/71-DATSUN-240Z-260Z-280Z-BRAKE-JUNCTION-CONNECTOR-BLOCKS-/170860083474?pt=Vintage_Car_Truck_Parts_Accessories&hash=item27c80df912#ht_500wt_970

Blue,

Thanks for the info. I had copied the part number as 46100-US200 from the schematic vs 46100-U5200. I have the one that was removed from my car during the tear down, but it was really dry and gummed up. I took it apart and cleaned all the parts but was not sure the switch would work. The image you attached will help me get it back together. I did pick up a couple of assemblies last weekend at a junk yard.

I read in one thread that the unit was supposed to be replaced with a new one vs replacing parts per the work shop manual.

I do have a couple of proportioning valves and three way connectors.

Wally Dill

I have read that many refurbished their switches. I had to use two to make one good one due to corrosion.

Thanks for the last picture, it will help me put it back together. Does the inner chamber ever have brake fluid in it?

No, it is not supposed to have fluid in it. ... but I think the o-ring in the centre contact is there for a contingency of a leak into the centre chamber.

The "o-rings" in the "dumbbell" keep the right side and left side fluids from mixing.

The dumbbell slides left or right depending which side looses pressure.

The contact hanging in the middle chamber (not shown well above) touches the metal part of the dumbbell and shorts to ground through the dumbbell and turns on the brake warning light.

Edited June 19, 201212 yr by Blue

I have read that many refurbished their switches. I had to use two to make one good one due to corrosion

There is a place that will re-sleeve brake components with stainless or brass inserts called "White Post Restorations". Quality workmanship and better than new when they are done with it.

http://www.whitepost.com/brake.html