Why are the rear brake lines 1/4 inch?

[vacca rabite]

All the other lines are 3/16.
Why are the rear lines 1/4?

Is there a reason not to use 3/16 brake lines there too?

Zach

[george walker]

I expect it was intended as a way of metering the pressure to the rear brakes. On most vehicles the rear brakes do most of the work, and a porportioning valve is used to meter the fluid pressure, the 715 didnt have such a valve, so I guess they just made the lines smaller in the rear. It would be best to keep the orignal line sizes I would think.

[Team Nefarious]

I always thought that most vehicles (cars) due to weight distribution and weight transfer the front brakes were the larger and the primary stopping force. The rear brakes being smaller to equal in size the secondary stopping force. And that trucks are assumed to be carrying much more weight on the rear and have more balanced to larger brakes in the rear to help stop the "load". The M-715 seems to have identical brakes front and rear (from the specs in the tech manual). If there is no proportioning valve I would assume that all four brakes receive the exact same volume of brake fluid. The rear lines being 1/16" larger wouldn't make any impact on brake force to me as the volume of the fluid moving through the lines and driving the brake cylinders should be constant.

[brute4c]

I seem to remember chnging all mine some years ago and they were all the same...could the rear lines have been replace on yours already or is my memory faulty?

[randyscycle]

The rears are 1/4" from the "T" to each wheel.

I suspect it was really only done because on the front the rubber hoses screw directly into the wheel cylinders and the wheel cylinders use 1/4" tube thread. To keep consistent and be able to utilize the same wheel cylinders on all four corners, I beleive the choice was made to go with the 1/4" lines.

As far as it goes on most vehicles, 80% of your braking ability lies in the front wheels, regardless of disc or drum. If you send too much pressure to the rear, they will always lock first, particularly with a truck as it is lighter in the rear. Remember the older GM and Ford pickups with the load compensating valve on the back axle? A larger line will not result in more or less pressure anyway, only more volume of fluid. The pressure is a constant no matter the line size. The master cylinder, or any metering device is the only thing that could make a pressure difference.

In early ABS cars, a pump and nitrogen gas accumulator system was used to generate the high pressures needed in reserve to actuate the anti-lock feature by pulsing the fluid pressure to the affected or less-tractive wheel.

[george walker]

Randycycle is correct that the rears are 1/4 because the front hoses at the wheels cyls are and its keeps the system balanced. I researched and relearned about brake balance. In the days of all drum w single circuit systems no proportioning was used, the brakes being drum at each wheel usually didnt require any compensating. When disc brakes came along (and dual circuits) proportioning was needed since it takes much more pressure to apply a disc brake than a drum brake (due to self energizing of a drum brake) so it was necessary to regulate the pressure to rear drum brakes to keep them from locking up, from the increased pressure. It was done in a couple different ways, some were porportioning valves others were more of a delay valve. An for a while trucks did have the load sensing proportioning valves on the rear to allow different levels of control with various bed loads. Modern anti lock systems have allowed an even better means of brake balancing.