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[Datsun 1200 encyclopedia]

Brake Proportioning

From Datsun 1200 Club

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Category: Brake System

Front and rear caliper/cylinder ratios should match the application of the car weight bias, brake and tire sizes. Otherwise, a brake proportioning valve will be needed to prevent premature lockup of the rear brakes (the valve reduces pressure to the rear brakes). Early Datsuns used fixed proportion brakes (with no valve), but in the 1970s some were fitted with a Load Sensing Valve Assembly called the NP-Valve. This allows good braking whether fully loaded or empty. The 1200 got a load-sensing valve in October 1989.

Contents

Overview

A proportioning valve is installed in the rear brake line to reduce braking efficiency and prevent early rear-wheel lockup. Datsun 1200 doesn't use one -- instead, the front and rear brakes are carefully engineered to provide the correct proportion.

The ratio of the brake piston diameter to the master piston diameter gives you the mechanical advantage.

But what about the front/rear proportion? You use a smaller mechanical advantage on the back to get the brake force distribution right. You want the front to lock slightly before the rears, and you can use this to tune the brake balance. This rule hold for all hydraulics by the way.

The same size caliper will stop the car faster with larger rotors. So caliper size alone doesn't determine the balance.

Factors
* Rotor diameter
* Drum diameter
* brake pad/shoe material
* Front/rear tire size difference (if any)
* Caliper bore size/wheel cylinder bore size

Start with your desired pedal pressure (i.e how much foot force you want on the pedal), pedal ratio and work out the required master cylinder size. Lastly, you need to ensure that the stroke of the master cylinder can move enough fluid for the brake size, so work out the total displacement of fluid required. The government regulations specify the maximum leg effort required. Nissan already worked this out by providing the 11/16" master cylinder (easier to press than a larger 13/16" unit, and generates more pressure than a smaller 5/8" unit).

But what about varying loads? A heavily loaded car, with weight in the trunk, will need more proportion to the rear wheels than a lightly loaded car.

Make a full-braking hard stop:

  • If both front and rear tires lock up at the same time, this is optimal for braking efficiency. For a margin of safe handling, the front should lock up slightly before the rear.
  • If the rear brakes lock up before the fronts, squirrely handling is the result. In this case an Adjustable Brake Proportioning Valve can be added, to cut down on the rear pressure until the fronts lock up first.
  • If the front brakes lock up long before the rears, the car won't be stopping as quickly as it should. Adding a Brake Proportioning Valve won't help fix this problem. What you can do is add more tire to the front (e.g. wider tyre), or use larger wheel cylinders on the front (or smaller on the back) in order to make the brakes more rear-biased. Or, changing to a less aggressive front pad will move the bias rearward and thus enable quicker stops.

The other issue with proportioning for a street car is due to weight transfer: Less than a full-braking-force stop will transfer less weight to the front, so more rear brake force can be used. For this reason Brake Proportioning Valves (both factory and aftermarket) don't start restricting the rear brakes until a certain hydraulic pressure is reached. This is what is "adjustable" in some of the adjustable valves, and so won't help with full-force braking.

Adjustable Brake Proportioning Valve
Wilwood 260-12627 10mm fittings
$45 USD

A Proportioning Valve is inserted into the rear brake line, and so can limit the rear braking force. WARNING: Never install on front brake lines.

B110

Datsun 1200 sedan/coupe/wagon does not use a brake proportioning valve. Instead the front and rear working cylinders are sized to achieve a good balance. The maximum weight loading of the car is restricted to 600 pounds passengers + cargo.

Compared to Datsun 1200 drum brakes, the disk brakes require a higher line pressure (harder pedal push) to make them work & this means that the rears would have a greater tendancy to lock up, so the factory fitted the rear wheel cylinders with a smaller bore size to balance the brakes out. Thus a correct proportion is achieved, which is different from the all-drums proportion.

If you fit stock 1200 type disc brakes, then modify or change the master cylinder (remove the RV from the front circuit) AND replace the rear wheel cylinders with 11/16" disc brake units.

All Drums 
Front: 13/16" wheel cylinders
Rear: 13/16" wheel cylinders

Disc/Drum Front
Front: 1.894" caliper cylinder with 212.5 mm rotor 
Rear: 11/16" wheel cylinders

Not to be confused with the Brake line pressure differential Warning Light Switch
7663.jpgAlbum 7663

Brake Upgrades

So you've changed brake types, now has the front brakes are locking up well before the rears come into play. How can this be cured?

Changing master cylinders may change pedal feel but will not change bias. A master with larger diameter cylinder will result in a harder pedal, but less pedal travel. By contrast, a smaller diameter master cylinder will reduce required pedal foot pressure, but will increase pedal travel for a given result. Most Nissan master cylinders from the same era (1971-1982) interchange. You can locate almost any size with a cruise through your local junk yard (sizes are cast on the side of the cylinder).

There are options for changing the brake bias:

  • Change rear brake wheel cylinders (larger or smaller). Alternate sizes are available from Nissan motorsports. Or from stock applications. For example, the 510 rear cylinders are 7/8, but bolt in place of the stock 1200 13/16 cylinders. Note that 1200s with drum brakes already have 7/8 rears.
  • Changing master cylinders will not change the bias
  • Use an appropriate factory bias valve (aka NP-valve or "proportioning valve"). Note that Datsun 1200s don't use one but B210/B310 did.
  • Maybe the easiest would be to buy a brake bias controller valve.
    • These are available thru your local high performance shop or mail order thru Jeg's or Summit, etc. for around $50 USD.
    • Some Volvos also have an adjustable valve you could use

Wilwood adjustable valve
20090809104902DSC08156.jpg

NP-Valve

The NP-valve used with USA B210 and USA/JAPAN B310 is integrated into the splitter and so serves as the circuit pressure loss safety switch.

It is functionally normally if -- during a 50km/h quick stop -- the rear the wheels lock simultaneously with front wheels, or front wheels lock first. Danger if rear wheels lock first.

Location of NP-VALVE in B210
23093.jpgAlbum 23093

Specifications

See: Brakes#Specifications

Part Numbers

46234 CONNECTOR [brake splitter]
46224 BRAKE INDICATOR SWITCH [combo splitter+switch]
46400 LSVA
* 46: Master cylinder, pedal &, piping group
* 400: Load Sensing Valve Assembly

Japan

B310

All use NP-Valve except some A12 DX/STD models which use only a splitter.

46400-F2300
46400-F2310
46400-R3001
46400-R3011

Late Sunny Truck

B120 Sunny Truck
46400-G1500 VALVE ASSY 8910-
46405-G1501 BRACKET-VALVE
08630-81851 BOLT
Bolts to Lower Back Panel
22477.jpgAlbum 22477

PB110

OPTIONAL for PB110 was a Proportioning Valve. It was placed in the line to the rear brakes.

46400-A1100 "P" VALVE ASS'Y (op.) DX
* Same as JDM Datsun 510 1970-1972 
* "ASS'Y-VALVE, proportioning (op.) S&C" 
* In the 510, it replaced the 2-way connector which all 510s have
46400-H1005 "P" VALVE ASS'Y (op.) GL,GX
46364-H2600 JOINT-union GL,GX 2
p-valve.jpg
S30 (240Z) to 0672 USA 46400-E4100 <> 46400-E4101  
p-valve_2.jpg

North America

B210
46400-U0506 ASSY-NP VALVE 4WAY SWI,FRA,GER,SWE DEN,NOR,BEL,HOL,L UX
* also used by early 610
46400-H7200 ASSY-NP VALVE 4WAY 0974-0777
* also used by A10 wagon
46400-S0801 ASSY-NP VALVE 5WAY 0877-
* also used by S110 0381-
B310 
46400-W5000 VALVE ASSY-NP 5WAY EUR,COM(A14,A15) OP COM(A12)
* also used by 910 0780- S
610 
46400-U0506 ASSY-VALVE NP, 4 WAY -0775
46400-N3700 ASSY-VALVE NP, 4 WAY 0875-
A10
46400-N3000 NP VALVE ASSY (4-WAY) SEDAN & HATCHBACK
46400-H7200 NP-VALVE ASSY WAGON
620 starting 1976, about 1/2 way back
0276- 46400-B1900 ASSY-VALVE LOAD SENSING (L.S.V.) DX
0875- 46400-B1900 ASSY-VALVE LOAD SENSING (L.S.V.) Truck
0876- 46400-B1901 ASSY-VALVE LOAD SENSING (L.S.V.)
0877- 46400-B9800 ASSY-VALVE LOAD SENSING (L.S.V.) $667 USD
720 VALVE ASSEMBLY [also called load-sensing valve, under bed]
2WD
0679-0681 46400-05W00
0781-1181 46400-03W00
0781-1181 46400-05W00 HD/T
1281-1282 46400-34W00
0183-     46400-10W00 DUAL WHEEL 46400-10W10
0183-     46400-20W10 
0183-     46400-81W00 HEAVY DUTY 46400-81W10
4WD
0679-1181 46400-45W00
1281-0785 46400-30W00 <> 46400-30W01 <> 46400-30W10
0885-     46400-30W11
 
46400-45W00
46400-45W00-2.jpg 46400-45W00-3.jpg 46400-45W00-4.jpg
46400-03W00
46400-03W00.jpg 

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