Can Anyone Explain the FJ40 Tandem Brake System to a Dummy?

Joined
Jan 1, 2011
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Tempe, Arizona
I'm trying to understand the FJ40 Tandem braking system and why the smaller reservoir and outlet (with the proportioning value) runs to the rear drum brakes and the larger reservoir and outlet appears to be supporting the closer front disc brakes (on my '78 FJ40).

It just seems weird that the smaller reservoir seems to support the rear drum brakes which are the furthest from the MC.

Is there an explanation for how this Tandem set-up is engineered?

Thanks for helping with my understanding.
 
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Dec 29, 2005
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Craig, CO
Realize on any brake master cylinder the reservoir is split in two. The early Toyotas have two separate reservoirs, but the later masters with one reservoir are just providing fluid to the two circuits in the same way. If you look into the reservoir from a 60 or 80 or pickup, 4runner, there is a wall inside that separates the fluid for the front and rear circuits creating two reservoirs.

The front circuit, either drums or discs is larger, as even the front drum cylinders were bigger than the rears. The front circuit as mentioned above does the bulk of the braking, and will therefore generate more heat. More fluid movement in system and more reserve capacity helps with heat dissipation. The master also allows for a bit of delay between one axle to the other, similar to the way you actually hit the rear brakes on a bicycle a few milliseconds before you mash the front brakes, so you don't lunge forward.

The other aspect of brake system design is failure safety. Independent reservoirs assure that if a line or cylinder ruptures on one axle or the other, you still have one complete axle worth of braking power to stop.

The proportioning valve is another step in this safety chain. Earlier systems used brake capacity and bore diameters to limit braking force on the rear axle, so the rear axle theoretically would not out brake, or lock up more quickly than the front end, which causes fishtailing and crashing in emergency stops. The brake proportioning valve located below the master has some valving in it, that limits rear braking proportionally to the force generated on the front circuit. If the front circuit does not generate any pressure when applying the brakes, due to a loss of fluid, then these valves can then open and allow full generated braking force to go to the rear brakes during an emergency stop.

On later models, like the 62 and 80 this system was further refined to include a weight sensing proportioning valve on the rear axle, that further limits rear braking force, if the rear springs unload, or allow more rear braking if the rear end of the vehicle is heavily loaded, and the traction is present to allow the greater braking force. It also utilizes the ability to force full braking power to the rear end if the front circuit fails. Load valves are common all almost all vehicles now, but of course ABS and VSC has even made that obsolete, because now a computer can decide when and where to apply the brakes and with what force to keep the vehicle in a straight line.

Before proportioning valves were put in the system, the tandem master had pressure switches on each circuit that would illuminate the E-brake light if there was a loss of pressure when applying the brakes to warn you to try and get stopped quickly, as you did not have braking pressure on one circuit or the other.
 
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Olathe, KS, USA
I'm trying to understand the FJ40 Tandem braking system and why the smaller reservoir and outlet (with the proportioning value) runs to the rear drum brakes and the larger reservoir and outlet appears to be supporting the closer front disc brakes (on my '78 FJ40).

It just seems weird that the smaller reservoir seems to support the rear drum brakes which are the furthest from the MC.

Is there an explanation for how this Tandem set-up is engineered?

Thanks for helping with my understanding.
While the others here have attempted to answer your question and have somewhat gone off the rails into other descriptions, it does not directly answer your question as to WHY the front reservoir is larger.

It has entirely to do with hydraulic volume.

The front wheel cylinders are larger than the rear wheel cylinders. It takes more volume to move them the same distance as a smaller diameter cylinder.

The front slave cylinders must have a longer throw than the rear cylinders because the drums are larger in diameter as well.

So, the front wheel cylinders are larger in diameter and have a longer throw than the rear wheel cylinders. It order to have them move at approximately the same time , or linear speed, the hydraulic VOLUME must be larger to move the front cylinders at the same rate as the rear ones. The front ones will be placing much more pressure on the drums because of the increased volume and the increased surface area of the brake shoes , drums, and volume of the wheel cylinders.

On many vehicles, the rear brake lines are smaller than the front ones as well because a larger diameter line will allow more flow (volume) in the same amount of pedal travel.

There's lots of math involved dealing with the conversion of hydraulic energy and force to change it to linear force within a given time frame.

These volumes are why it is necessary to change the master cylinder as well when changing from drums to discs because discs have MUCH larger pistons, but a much shorter throw and the braking hydraulic system must be balanced in it's pressure, volume, and braking pressure at a given time.

Look up Poiseulle's Law as well as Bernoulli's Equation.

The piston inside the master cylinder also is not equal from front to rear, so there is a volume difference there as well in order to compensate.

It's all about hydraulics. Flow, volume, and pressure. Not heat or cooling.
 

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