Clarification on this: The check valve is required only on drum brakes to keep enough pressure on the wheel cylinder cups from collapsing and leaking fluid. You cannot have a check valve on a disc brake wheel cylinder or it does not retract enough and the shoes will drag causing real issues. So systems that have both discs and drums have a master cylinder that has only one check valve for the drums only and the other half has no check valve.ImperialCrown said:Plus there is residual brake line pressure in drum brake systems to keep the wheel cylinder rubber cups flared out against inside the of wheel cylinders. It isn't enough pressure to overcome the shoe return springs, so the shoes aren't applied and don't drag. It is however enough pressure to keep the disc calipers partially applied to drag, heat up and wear.
You want to do a full proper conversion as safe brakes are paramount. The AAJ brake kit as shown in the post #3 link is expensive, but complete except for the metal lines which you can cut, form and flange yourself with a 25' roll of brake line and flaring tool.