just curious because i had to have my entire brake system changed out. On a 280z why is there a second brake proportioning valve on the firewall for the rear brakes when pn all other models there is only one proportioning valve? i ask this because that valve of mine was broke hence why my brakes failed and when i tried to get it from nissan they said it was discontinued but the other valve wasnt. And also 240z have the same exact srtup for brakes and they only have one proportioning valve which is the same as the first 280 valve so why the second valve?

Actually there is only one proportioning valve on the 280Z, just like there is only one on the 260 and 240.

The confusion you have is that the master cylinder connects first to the brake failure switch, then the rear brakes connect to the proportioning valve.

On a 240Z the proportioning valve is located behind the right rear wheel. On the 260Z and 280Z it is on the firewall.

I have the valve from my 240Z here at my desk. The stupid thing leaks like a sieve, and I can't find another one.

So far the best that I can find is an adjustable unit that Motorsports sells for their rear disk conversion kit. If I end up having to go that way I will mount it in the engine compartment so that I can tweek it until I get the balance right.

I do not think that the item in the rear (near RR wheelwell) is a proportioning valve at all.

I don't have my shop manual with me, but Vic Brit describes this item as a "3-way tube connector", which is what I believe it is, a connector and nothing more. What could it possibly "proportion"? RR and LR wheel cylinders at most. It has no connection to the front brakes, which do "most" of the work. Also, this same little item (I call it a distribution block) appears on 70 - 78 Z cars. Vic Brit lists the same part for all of these years, PN #64-627 for $16.95

The item on the firewall of 73 - 78 Z cars is called a "4 way valve assembly" and BOTH front and rear brake tubing connects to this item. This would fit the description of what I would call a proportioning valve. I do not believe that the 70-72 Z's had a proportioning valve.

I'll check my FSM for more info tomorrow. Anybody have their FSM available now?

On my 71 there are 2 "items" in the right rear wheel well.

One of them is a three way tube connector, what I would call a "T" fitting, and the other is called a "pressure regulator" by the Hanes manual.

When I called the local Nissan dealer asking for a proportioning valve for my 240Z he came up with a part number... which is not available.

It may be that the newer cars have a more sophisicated proportioning system, but this thing must be doing something...

It appears to have some kind of spring loaded metering rod or something inside. If I don't need it, that is ok too, but I really doubt that.

Walter:

Do you have a digi camera to post a picture of the item you're refering to? I don't recall anything but the connector at the rear of my car (that doesn't mean it isn't there). The Vic Brit catalog doesn't show anything else in the system back there, and their drawings are usually complete even if they don't have the part.

Still, I cannot imagine what significant value something connected only to the rear brakes would have on a street car.

I'll check FSM and Haynes, etc tomorrow.

I don't know about your local Nissan parts guy, but mine is a fountain of misinformation. Sometimes I wonder if he even has a S30 fiche in that machine.

If I did this correctly, the two pictures attached show the "Pressure Regulator Valve" for the rear brakes of my 71' 240Z.

By the way, the Haynes manual uses the exact same term to describe the proportioning valve on the firewall of the 260/280.

The more that I look at this thing the more that I wonder... It appears to be completely sealed. There is one port that connects to the brake failure switch / master cylinder (Inlet) and the outlet connects to the "T" fitting that splits the lines off to the two rear wheels. There is no way that a totally sealed unit like this, with no return path could really regulate a hydraulic circuit.

At best it is a variable restriction in the line... perhaps just a surge suppressor? A device like this would keep the rear brakes from locking first, at least it would on drum brakes, since they are prone to locking on a sudden increase in wheel cylinder pressure. It might just give the front brakes a "Head start" in the braking process.

Thanks Walter. So this thing is "upstream from the T?

I'll crawl under my car tomorrow and see if I have one of these in my 72.

Walter:

Looked at the Vic Brit catalog again, and I do see the item you pictured. Not listed as available, and appears in the diagram in very light drawing, and only for 70 -72. Hmmmm.

How do they (Nissan) expect people to fix cars if parts are not available?

Um... perhaps they don't expect people to fix 30+ year old cars...

I am just trying to scrounge up enough information to plot a plan of action without their help.

Have you inquired at places like Eric Neyerlin's www.zparts.com and Danny's Datsuns at www.webspawner.com/users/dannysdatsuns/index.html They have had the hard to find parts when I needed stuff.

I just Emailed both of them, and I told them that I heard about them here in the forums. It never hurts to let people know where to look for potential business.

I thought that there was a long thread about this item just recently. I belive that the 3 way valve in the rear is specific to the s30 with rear drum brakes. It is to keep some back pressure on the rear brakes so that they do not need to travel so far. The thread also said that some people were disabling the 3way valve when they install rear disk brakes.