The Stovebolt Site Forums The Shop Area Driveline Brake advice

When you have only drum brakes, the brake fluid returns to the reservoir when you release your brake pedal. You release pressure on your brake pedal, the brake return springs causes the shoes the retract, which cause the wheel cylinder to retract, which forces brake fluid back into the system.

With disc brakes, that does not happen. When you apply the brakes, fluid fills the calipers and pistons compress the pads. When you release your brake pedal, the fluid does not return to the reservoir. As your brake pads wear, you need more fluid to apply the same amount of pressure.

That is why on a master cylinder for a disc/drum application, the disc reservoir has a larger capacity than the drum reservoir.

I think I've go that right, but if I don't, some knowledgeable Bolter will correct me.