disk brake conversion question

[pishta]

Went to 67 front disks on my '65 Barracuda. Got a dual reservoir MC in the purchase but it looks to be the drum/drum MC as both reservoirs are the same size. question: I know the disk MC has a larger reservoir for the disk side, but I dont quite get why the disk side would be bigger. If the brake pads are .06 away from the disk (or even slightly touching) how much fluid is transferring to the pistons to compress the caliper. If its more fluid transferring, wouldn't the pistons be bigger? I read (the theory) that it wasnt a transfer of fluid but just a pressure change that caused the caliper to apply clamping force on the disc. I know the rear drums actually expand the cylinders more so than a caliper as there is more play in a drum setup, but how does the tandem brake cylinder, working off a common piston rod (stroke) and similiar piston cup diameters give correct line pressure to 2 different systems? Seems that the caliper would build pressure a lot faster than the slave cylinders and actually limit pressure to the drum side, ie the fully compressed caliper would stop the M/C piston travel and would limit the amount of pressure that the rears would ever get. Is this by design, like the 80/20 split of a disk./drum brake system? And what about the tandem diagonal systems that the imports run, disk+drum. Where is the bias control? I know it works, just dont quite understand the mechanics.

[Ray Bell]

Some of your questions I can't answer...

But this one I think I do understand. With drums, there is an adjustment built into the system, usually working automatically. So that takes up the slack as the linings wear.

With discs, there is no adjustment, the pads just march on out progressively as the miles are piled on. So the fluid level must drop from new pads to old.

[Loadrunner]

Regarding the MC reservoirs you're correct in that at any given point in the life of the pads there is not a lot of piston movement to apply the brakes. However, with new brake pads the caliper piston is almost completely pushed back in the caliper to make room for the pads. As the pads wear the position of the piston moves farther out to take up the space from the ever thinner pads.

As for the different "pressures" in the brake system the answer is a little more complicated. In a closed system like the brake system, let's say you press on the brake pedal and the piston in the MC pushes on the brake fluid. The fluid doesn't compress so it moves the piston and applies the brakes. The "pressure" in the system is uniform. That is, if it's 1000 psi in the MC then it's 1000 psi in the caliper. The "force" is what changes. 1000psi, or pounds per square inch acting on a piston with 1 square inch of surface area develops 1000 pounds of force. Double the area of the piston and double the force developed by the piston. By engineering the size and movement of the pistons in the MC, brake cylinder and caliper you can get different forces and piston movement in the system.

This is my understanding of how the brake system works...if somebody sees something wrong please comment.

[cudabob496]

Should proportioning valve be discussed somewhere in this dialogue?

[pishta]

rethinking the question, I get the reservoir difference now. Sure, it gets progressively lower as the pads wear, so I can just keep the fluid monitored and use the drum/drum M/C? Working on diagonal brake systems from my past slew of cheap cars didnt quite jive with the "disk out of this one and drum out of that one" thinking. Im going to use a proportioning/safety valve out of a disk/drum latter model car so hopefully that will help in brake bias without an adjustable P. valve.

[aarracer]

The drum - drum master cylinder will cause grief with the front disc brakes. The drum type master cylinders have a residual pressure valve behind the flare seat to hold a minor residual pressure on the drum brake shoes. It will cause the disc's brake pads to drag.

[pishta]

AAR, excellent point! I had forgotton about the RPV inside the drum M/C. Can it be removed? What app had a manual disk/drum 4 bolt M/C so when I go to the parts store, I can tell them a year/make. Thanks, that saved me a few headaches.

[aarracer]

The residual valve can be removed by removing the flare seat with a spiral type thread extractor and vise grips. But it may damage the flare seat in the process. The 1973 dart has the MS you are looking for..

[pishta]

Cool, Ill give it a go. I got nothing to lose. 73 manual disk M/C looks good to me too if I have to go that route. Thanks again.

[John Kunkel]

Originally Posted by pishta

Sure, it gets progressively lower as the pads wear, so I can just keep the fluid monitored and use the drum/drum M/C?

Sure, most disc brake cars prior to '67 only had a single-pot MC.