I just got an 89 F700 for my business, and am wondering how the brake system works. It has a 23,100 GVW and hydraulic brakes, and has regular brake lines going to regular wheel cylinders, in the front side of the rear drums. That much makes sense to me.

For parking, it has a yellow pull-out ****, exactly like a heavier truck with air brakes. Now, I've heard something about some sort of electric hybrid brakes on some of these medium duty trucks, but that doesn't seem to be the case here. There is a large (probably 3/8" ID) air or vacuum line that goes to the rear axle and splits to each side, where it connects to a can that sits behind the axle tube, next to the drum. It doesn't look quite like the air brake chamber on a heavier truck, though, and I don't hear massive air movement when I use the parking brake, although it does hum a little bit. There are also a mess of air/hydro pumps and lines under the hood, and it doesn't have A/C, so I presume some of that may be related.

I was just wondering if someone could give me some sort of general overview of this system.

 

We have some 93 and 94 f600 and 700's at work too. They have the 5.9 cummins and a 6 speed manual, I haven't looked at the axle, but there is a seperate pump that looks like power steering that pumps something somewhere where you pull the ****.

 

Well, I'm still not entirely sure how they work, but they work well. I put 400 miles on the truck in the past 24 hours, and it performed admirably. Two more trips like that, and I'll have earned my $2750 back that I paid for it.

 

> I was just wondering if someone could give me some sort of general overview
> of this system.

Never have worked on a F-700, but, how it works for most larger trucks and tractors (trailers).

Big springs inside the cans push on the shoes, air pressure pushes back, once the air pressure falls down to around 40 psi, the springs overcome the air pressure and activate the brakes.

When you push in the **** you release the air from the system, thus, locking the rear parking park (as long as they are adjusted properly).

Some of the hoses and stuff under the hood prevent you applying the air brakes to the rear axle the same time as the hydraulic brakes. Plus, if you have a diesel with hydraulic brakes, you have a vacuum pump too.

Almost forgot, these are usually called "foundation" brakes and the most popular is the S cam (looks like a big S in the drum). I would get the manual so you know what is what, BUT, I would not work on them without training. The springs inside the cans (chamber) can go right through a wall.

FWIW: Only use a remote (25 feet away) air gauge and clip on air chuck to fill the tires. If a tire explodes while filling, you are dead if your head is anywhere near it.