Brake Discussion...

[Daryl Smith]

Irrelephant

[fj20spl311]

You are correct that the valve reduces the pressure to the rear, but not linearly. When added to the nonlinear response of drum brakes, you get much closer to a linear response that the disc brakes yield. They still need to be balanced front to rear.

[Daryl Smith]

Irrelephant

[fj20spl311]

By all accounts the roadster is balanced well from the factory with no proportioning valve....

There is no way for a disc-drum system to be balanced through out the braking range without a proportioning valve.

The torque curve vs pressure is a straight line with disc brakes. The torque for a drum system is a flat quadratic.
Very few if any American cars with Disc-Drum braking don't have both a proportioning valve and a delay valve.

[Gregs672000]

Not to hijack this discussion, but are said valves located within the master? I plan to switch to the 280z master as others have reported better braking with larger brakes and rear discs, which I have. Is this part of the change, a master set up better for rear discs (which I assume the 280z had. Haven't looked.)?

[fj20spl311]

@Greg672000, the valve is a stand alone unit. The inherent need for a Proportioning valve is ONLY for a Disc-Drum system.

@Daryl Smith, My suggestion is don't over-think it, just install one on your existing system to see how it reacts.

[Gregs672000]

Thanks Phil...

[fj20spl311]

One way to think about it is disc brakes produce lock-up usually say 1000 psi, where drum brakes are under 800 psi.
The valve reduces the line pressure so both systems get to their lock-up pressure together.

[JT68]

The rear brake situation is not too hard to manage in practice. As Phil said the most elegant method is a proportioning valve, the objective simply being to give the rears as much pressure as possible, but still prevent the rear brakes from locking before the fronts. Its pretty apparent to the driver and easy to spot externally if the rears lock first.

Going to a larger master cylinder LOWERS the hyd pressure which will make the rear brakes less effective. if using larger pistons upfront, the fronts will be more effective, keeping things as it should be, and keeping the car from getting tail happy. Upsize the rear cylinders if you need more in the back (13/16")

If you really want to optimize the rear brakes, rear disc conversion with a valve is the way to go, but for most street uses that is unnecessary, unless the car is stupid fast, like sr20det etc.

As was correctly pointed out earlier, you want the smallest MC that still moves enough fluild without excessive pedal travel. Too big of a master will give a nice firm pedal, but decrease your braking performance.

Also, just a note on the Tandem cylinder, it actually does have a crude front/rear bias system- its the spring in between the pistons. Under light-mid braking the rear piston pressure is controlled by the spring, not the pedal directly.
Unfortunately with a panic stop (when you need as much control as possible) that system lets you down because the spring may bottom out and provide full force to the rear. (Not a great design, but hey it was 1967 -so state of the art)

That also may be the reason the single master OFTEN FEELS better ( more contribution from the rear) during around the town driving.

I realize of course that the dual master is considered a safety feature, But fortunately most brake failures involve slowly leaking fluid etc, not catastrophic failure. For normal driving honestly i wouldn't be super scared of the single master ESPECIALLY with new modern calipers where the likelyhood of total brake fail is minimal. 99% of the time you will know from your pedal, peeling paint or a small drip somewhere if you have a hydraulic leak. The other 1% the emergency brake works just as well as the compromised hydraulics,maybe better. (From experiance lol) if you were super paranoid about it you could use a later model 510 master cap that has low fluid sensor and use that to light up a light- again not necessary for a true enthusiest. (Prob a great idea if your kid/loved one drives the car frequently)

For road racing a single master is unacceptable of course- gotta have adjustable duals.

280z was rear drum, ZX was rear disc so the masters are likely different internally FYI , j

[fj20spl311]

Also, just a note on the Tandem cylinder, it actually does have a crude front/rear bias system- its the spring in between the pistons. Under light-mid braking the rear piston pressure is controlled by the spring, not the pedal directly.

I had always heard the spring was there to compensate for the differences in volume, its had to believe its strong enough to compensate for different pressures expect at very low pressures.

That also may be the reason the single master OFTEN FEELS better ( more contribution from the rear) during around the town driving.

[JT68]

I've never measured the working pressures directly on either type OE cylinder, but think the working pressures would be similar otherwise the late brakes would be absolutely terrible compared to the single master cars. (But they aren't)

I think we were just speaking apples and oranges, cause I think you mean the pressures would be half with a DUAL master setup (so you would use smaller cylinders to compensate) , but with a tandem system the second cylinder is just a slave to the first, so I think the pressures would be the same in both circuits unless that spring has some effect, or otherwise designed to be different. (At least in my brain lol)

Either way, I totally agree a manual proportioning valve is a very good idea for tuning the rears if significant changes are made to the system since it is a pain to swap out rear cylinders once they are in there.

Cheers! J

[Daryl Smith]

Irrelephant

[Daryl Smith]

Irrelephant

[redroadster]

The dual Datsun MC has a proportion ing valve it applies rear fluid then frts
This modification isn't allowed for street , insurance coverage

[JT68]

Back here again....

Anyone have pictures of, and/or information on the 'twin' single master cylinder setup? Looks like I am going this route with my low windshield car.

Also, if going to the twin single master, does a reduction to 5/8" from 3/4" make sense? Less volume would be needed since there are now only the
front or back brakes to contend with for each cylinder, which would increase the pressure/lessen the force required to brake......
More calculations...

Thanks....

Hi Daryl, sorry, I just saw this one. I'd suggest staying with 3/4" (at least as a starting point) for two main reasons:

a. the wheel cylinders are designed for the working pressure developed by the 3/4" with the stock pedal ratio. Not sure what the maximum safe pressure is for the Nissan calipers/cylinders. (I've never seen it published, but Wilwood for example does provide a max about 1200) The 5/8 might exceed that, don't know, you would have to test it.

b. since you will have two cylinders, each one is only moving around 1/2 the fluid to engage the brakes that the one master used to move...so you will have a firm pedal with the 3/4's and again perfect working pressures assuming the same pedal ratio. If you used a 5/8", significantly more travel would be required to move the same amount of fluid. (you are correct- higher ultimate pressure would be reached with the 5/8, but the trade off is more pedal travel to get there)

An 11/16" might be an option if you can find a pair, but again I don't know what the max working pressure would hit in a panic stop situation...pretty high.

Pedal ratio is very important and in general, it is a lot of work to retrofit these to a production car so typically just for racing:



Really firm/strong mounting points are needed or else the assembly will just flex/fatigue the firewall. Good luck and have fun! j