Just about have all the parts I need for the disc brake swap, got the parts from a 73, disc no booster and 79, disc with booster.
Two things are in question, I got the prop. valve from both trucks, one is a small simple H block from the 73 that is the same as the one for the drums and mounted in the same place. The other is about twice as big but mounted on the cross member, instead of the frame. Can I use either one, or are they different internally because of the booster? I'm not sure whats going on inside them as far as port diameters and such.
The other question is about the booster. I just replaced the drum booster, which is smaller diameter, two months ago. Can I use it for the disc's or should I take the core from the 79 in and get the larger booster? How much does the difference effect the braking, is it unsafe?

Again Thanks
Jim

 

Jim: Your post will generate a lot of response as this is a favorite conversion on these old trucks. The proportioning valve (PV) does a couple of things besides provide a place to “Y” out all those lines. Drum brakes are self energizing and require a little residual pressure to operate properly. This is why drum brakes are a little firmer on the second pump. The space between the shoes and drum is taken up. A valve in the PV maintains this residual pressure. The big PV also has a shut off valve that isolates part of the system if it detects a loss of hydraulic pressure. This can be a pain when bleeding the system. The valve senses a pressure loss (cause you got the bleed screw open) and shuts that circuit off. You will have to work through this because these valves do not act the same. I’d use the big PV and the small booster. Booster is easy to change so you may want to try both. The hole in the brake pedal arm that accommodates the booster/master cylinder actuating rod will need to be relocated down about 1/16 inch and forward about half way between the existing hole and the edge of the pedal. Fail to do this and the pedal will sit just about on the floor. Also the pedal arm return stop will require some modification otherwise the pedal is stopped by internal components of the booster or MC. I would mount the PV down on the frame under the cab. Run the two MC lines close to the cab, close to each other and connect to the PV out of sight under the truck. Things get busy around the MC/booster area and the more room there the better. Also the PV is a low maintenance item that works in a dirty environment, so access is not really important. If you’re careful you can use the original rear brake line by replacing the fitting and flaring the line. There are lots more details related to this swap. Please keep us posted!
William in Atlanta