C30 Rear Brake Line Size on a 157 wheel base.

[david7w5@gmail.com]

I have a 1972 cab n chassis c30-157 wheelbase which is the longest wheelbase available in that year model. I am replacing some brake lines on the rear axle of a Dana 70. Everything that I have seen online is showing 3/16 fuel line but the truck appears to have 5/16 on the rear. I look at the proportion valve and the front lines appear to be 3/16 and the rear line appears to be 5/16. Is this common on the cab and Chassis or has somebody changed the brake line. It does to Pure Stock due to the rust and the routing of the brake line. When I search in line tubes online all they show is 3/16 fuel line. If you use the 3/16 fuel line that everybody shows online will that affect the breaking of the truck?

[Sheepdip]

They used 3/16" lines with 3/8"-24 inverted flare nuts. It is possible that, from the proportioning valve back to the junction block/tee on the rearend they ran 1/4" then 3/16" out to each wheel/drum.

3/16" will not affect the braking, brake fluid doesn't compress.

[kwmech]

The factory HO72 that probably came in the truck would have had 1/4'' lines on it. The Dana 70 could have either depending on the wheel cylinders. I remember I had to get a little creative when I swapped out my HO72 for a dana 70 hd

[david7w5@gmail.com]

Thanks

[RichardJ]

I'd like to add to post #2 above
Sheepdip,
>>, brake fluid doesn't compress.<<

A brake system doesn't transfer fluid from one point to another. It transfers pressure. When you apply pressure to a volume of fluid, the pressure is the same in every area of that fluid.
If you apply 100 PSI to the fluid at one end of a tubing in a closed system you get 100 PSI at the other end of the tubing. Little to NO fluid has to move to transfer that pressure, because the fluid doesn't compress. Very little fluid transfers to move a disc caliper .003" . Drum brake shoes, especially if out of adjustment, will take a little more.
Tubing size has little effect on the transfer of pressure.

Fuel flowing in a tubing will have a TIME element. A fuel pump will be rated with a specific VOLUME in a specific amount of TIME (gal per hour). Tubing size only becomes an issue when Volume x Time, requires increased pressure, because of small size.

Tubing size could possibly play a roll in ABS brakes, because they do have a TIME factor. They constantly adjust the pressure in microseconds. But, the ABS controller has a pump to restore pressure to the hydraulic system after the ABS valves have released pressure.

[PbFut]

Richard is correct on movement of fluid relative to pressure. There is some fluid movement or the pedal would feel like a rock all the time but it is small. It mostly has to do with pad or shoe kickback. It is why you see really good road racers pump the brake before a critical turn. This way they can heel toe rev match shift and know the brake and throttle are in position. And no pedal to the floor surprise as the out side wall quickly approaches. It sounds simple, but it is very difficult to learn the technique with smooth results. Unfortunately, today's padel shifting trannies take much of that ballet away. Not to say left foot braking and padel timing is not difficult to get right. Something this old dog would struggle to unlearn and relearn. Can you tell, I miss the track?