AE86 proportional valves (brakes n shizno)

[Jonny Rochester]

The one piece and the two piece items both function the same way. There is no internal connection between front and rear fluid in these items. The working differences is the calibration of the proportioning valve. It is a generic p-valve body that Toyota have for most vehicles, and it works the same for disk or drum. The engineers obviously select a setting for that vehicle. The valve is not a one-way valve, and it does not regulate volume of fluid. A graph has to be used to show the exact function of it. The turning point, or refracted point in the graph, is what is used to name the different valve settings. When the fluid gets to a certain pressure, the valve starts reducing pressure to the rear in proportion to the master cyl pressure.

For the vehicles we are interested in, the proportioning valve turning point is set at:
AE86, ADM = 15kg/cm^2
AE86, JDM GT with drums, Euro GT with disk = 20kg/cm^2
AE86 with disk rear, AE85, some Euro = 30kg/cm^2

It is worth noting that these same valves come on a host of Toyota vehicles, including the 1977 Mini made in Australia. Also, the AE85 (with drums) uses the exact same p-valve as the GTV and GT-Apex models with disks.

If you have a "ADM" AE86 and just swap to the rear disks, and want to keep everything factory, you can use the p-valve off the RA40 Celica (just one of many vehicles). That is the exact same valve the disk brake models in japan had. So there you go... (I should be getting money for this... oh well).

If you have larger front brakes (and rear disks), or if you don't want to find a different p-valve, you can just gut your valve so it has no effect. Rather than just removing the spring, I think you should remove everything you see in there unless you figure how it works.

[n00bvak]

man jonny, how the hell do you find out this stuff???

*amazed*

[af300e]

He actually reads playboy magazine for the articles.

[rthy]

Thanks Johnny, exactly the info i was after.

[sundee]

hey just thought i might add to this, when doing the rear disc conversion on my dato 1600 from drums to R31 disc's, The preasure relieve valve inside the rear section of the BPV had to be removed, this had something to do with it not allowing the release of the rear breakes propery or quick enough when u stopped breaking.. this pic posted on the 1st page.. all i had to to was remove the spring and that tube and put it all back together.

[Jonny Rochester]

This shows a typical 1-piece proportional valve, like some KE30s had. At No.3 you can see a shuttle valve, which makes it appear that the front is connected to the rear lines. But this shuttle valve is there only to operate a safety switch, and light on the dash, if the rear brake pressure (from the master) should be different to the front brake pressure. The AE86 does not have this.

Law states you need a brake failure light on the dash. With the AE86, there is a fluid level switch on the brake fluid lid, which does this job. So a switch at the distribution block is not needed, but the 1-piece design remained.

[rthy]

so what is the difference between the to? like how does it control the cm^2? spring rate?

ie. if u put the disk rear pv spring into the drum rear end pv it should change the cm^2 to suit disk?

[LittleRedSpirit]

I assume at that listed pressure, the spring starts to compress. This will increase the volume of the line to reduce the rate the pressure increases, as you increase pedal pressure? The fluid pressure still increases but at a reduced rate?

Its a tandem brake system designed to leave you one end if the other fails, so they aren't connected, and really shouldn't be. Any failures should hopefully be limited to front or rear.

[Kid Karola]

I remember when i fitted JDM front discs and disc rear-end, I also fitted a 2 piece P-Valve consisting of T-Piece for front and a seperate bias to rear (Can't remember the Part #'s) think it was somewhere on the old site, more than likely from Jonny's epic Part # thread!) I think it was either kouki spec or superceded the one piece part #.

[Sam-Q]

I want to know exactly what that spring setup does inside the bias valve bottom end. I had and still had the combo of ma61 disks and calipers on the front and ae82 calipers with 280mm disks on the back. With the stock master cylender and then later to an upgraded 15/16" cylender I had so little rear bias it wasnt even taking the machining marks off the rear disks. After I gutted the bias valve I found I had ideal brake bias.. win!