I've got a prostreet '68 that used to be drum/drum and was converted to disk/drum some time ago. The previous owner said that when he did this conversion the pedal was super soft. So he put in a adjustable prop valve until the pedal felt good. Pedal effort is very high and it locks the rears. My guess is that 90% is going to the rears and 10% front.

I guess my question is: Are the master cylinders the same for a disk/drum car as for drum drum. It would seem to me that by adding the front claipers, piston size tripled and requires much more fluid...hense the soft pedal. I think I need a MC with a bigger bore. And btw this is manual brakes. thanks.

I do know they are different - but I'm not sure where those differences lie.

The master cylinders are the same for Drum or Disc. But there is a check valve that goes between the master cylinder and the brake line going to the disc setup. This check vavle holds pressure in the line because the disc brake caliper pistons needs to have a pressure on them or the rubber seal pulls them back into the calipers some. Hence the soft brake when you push it.

Change to a quick take up design to eliminate the need for the residual check valve.

FYI, all masters have a residual check valve, for drum brakes it's 2# and for discs its 10#.

Disk brake master cylinders have a larger front reservoir.

Where do I get said quick take up MC? Though that seems more like a band aid. The seal roll back is a function of the seal groove design (rubber twists and returns to shape). This is what allows the brakes to self adjust. If you press the brake and release with new pads it will return x mm's. If you do the same with worn pads it will be the same. The seal after so much travel allows the piston to slide. Keeping residual pressure in the system would counter act the seal roll back and increase drag torque, wear..etc on the brake during free wheeling.

What about the larger bore MCs? Is there a part number or prefered brand? Seems like that with my adjustable prop would do the trick too.

Never mind what I said, I was wrong.... graemlins/clonk.gif

Make sure the adjustable prop valve is working properly. Make sure it's spliced into the rear lines and not the fronts!