why only 280z

[dogma420]

on stock brakes for the 70-78 Z's, there are NO proportioning valves at all....Nissan designed the brakes so that the discs were proportioned perfectly with the drums, so without a valve, they are proportioned correctly.

[240ZX]

I believe the unit pictured by Walter is in fact the proportioning valve, at least for the early Z cars that I'm familiar with. Mine has the same unit. Carl, the valve may not be connected to the front brakes directly, however, the line pressure to the front brakes and supplied to the input of the proportionong valve are the same pressure. Why? Because the diameter, travel, and applied force to the pistons in the master cylinder is the same. The pressure out of the proportioning valve is less than the input pressure....hence the proportioning. For the rear disc conversion, just remove the internal parts from the valve, as recommended. Works great for a street application.

[Bambikiller240]

Sorry, I really can't buy that explanation. Why then is there another P. valve on the firewall of 73 to 78 S30's and the item we are discussing in the rear is still in the system? Two P. valves? Why? Doesn't make sense to me.

[ChrisA]

Here's a shot of my 72 with the twin valves. It looks like the main line from the front goes to the valve with two ports and then over to the "tee" which feeds each rear wheel. I always assumed the two port valve was a "proportioning" but now I dont know. I also thought the one below the master cylinder was the "proportioning" valve.

Chris

[PROJECTRB240SX]

That Looks Alot Like The Clutch Loop Damper On My 240sx... Its Used To Lower The Pressure In The Clutch Line For A "lighter" Pedal.

[280zgod]

well the piece im talking about on the firewall is a proportioning valve that cant be denied because i called nissan up about it and they said its discontinued good luck finding one in a junkyard. And i did but ive never seen the part you guys are talking about i guess only 240z's have that one and the one im talking about is its replacement part for the 280z. i dunno just a thought. thanks for the info everyone.

[Bambikiller240]

I checked all of my manuals, (FSM's, Haynes, Clymers, Henley, etc) and the item we were discussing (picture posted by Walter Moore) is indeed listed as a Proportioning (or Pressure Regulating) valve. Nissan recommended inspection interval is every 30,000 miles (50,000 km). The FSM has a detailed explanation of it's function. Basically it is just trying to make the rear wheels lock either "at the same time" or "after" the front wheels do so.

OK, so I was wrong.

[Walter Moore]

For what it is worth, the Haynes manual shows the item that I posted the picture of as the proportioning valve for the 240Z, and the one on the firewall as the valve for the 260Z and 280Z. I suppose that it is possible that early 73' 240Z's might have come with the former and later cars with the latter valve.

I sincerely doubt that any cars had both valves, as that would cause all sorts of problems.

Part of the confusion here is that all the Z cars 70 - 78 had the three port T connection, which looks a lot like the rear-mounted valve.

In any case, it looks like 280Zgod and I are going to be looking for an alternative since both styles of proportioning valves are no longer available.

As I posted somewhere... mayhaps in that other long thread... Motorsports and others who sell rear disk brake conversions also sell an adjustable model of proportioning valve. I am not sure if the adjustable unit will work with rear drum brakes. I guess I could try it and find out.

It looks like it is intended to replace the firewall mounted unit, so if I decide to go that way I will have to re-bend some of the new pipes that I just finished.

It is going to be quite a while before I get my Z back on the road anyway, so don't anyone hold their breath waiting for me to test on of these adjustable valves with the stock rear brakes.

[Miles]

There is a lot of mis-information here. The proportioning valve (PV) is connected to the rear brake circuit only and its' purpose is to limit pressure to the rear brakes so the front brakes lock up just before the rears. If the rears lock first the locked wheel acts just like a rudder and spins the car around.

Walt has a good picture of the stock PV valve above.

In the 72 240Z the proportioning valve is located just behind the passenger side tool box. The line from the rear circuit of the MC attaches to one side marked "MC" and there is an outlet rear wheels. It can not be repaired. As I recall there is another connecter (splitter) block back there too, which may be what VB is refering to. Although I always assumed VB was most likely refering to the PV valve.

I replaced my stock PV with an adjustable Wilwood PV mounted inside of the passenger side toolbox so I can adjust it from the inside. First I removed the stock PV valve. I used some off road SS flex hose and a few thread adaptors to connect the adjustable PV to the two rear lines exactly where they used to connect to the stock PV. You could also use hard lines to plumb the PV.

The reason I installed the adjustable PV is because my 240Z has a 350 SBC that put just enough weight forward that my rear wheels would slide during hard braking. The PV fixed the problem.

Note: Be careful and do not hook a PV up to the front brake circuit if you are going to mount the PV near the MC. Also, if you mount the PV near the MC be sure to gut out the stock PV by removing the large bolt on the bottom. Once the bottom bolt is removed the guts will fall out. Put the bolt back in and now the stock PV is just a connector block. You don't want 2 PV valves.

Miles

[Bambikiller240]

The VB illustration shows the 3 way splitter, and lists it as an available part. (see my earlier post for details). Their illustration also shows the P valve in very light drawing (compared tothe rest of the illustration) and absolutely no mention of it is made in the parts listing, though the bolt that secures it to the chassis is listed as available.

[Walter Moore]

They will sell me the bolt to attach the part that they can't get?

That is very helpful...

:laugh:

[240ZX]

I,m getting a real out of this post! There is no rocket science involved in the Z car brake system, it's just basic hydraulics being applied. I have to agree with Miles, there's a lot of misinformation being posted. What miles said is correct. Carl is have a hard time here, so many things are happening!? That OK buddy, we still love you and your brakes will work even if you don't understand the system completely. (I'm trying to humor you here Carl). Is everyone on the same page yet? :classic: