Disc Brake conversion power master cylinder question

My 69 Barracuda has an SSBC power disc brake conversion. It still has drum brakes on the back. It is a corvette master cylinder and a GM brake booster with an adjustable proportioning valve on the rear brake line. This was installed by the previous owner.

The rear wheel cylinders were leaking, so I have been replacing the rear brakes. I have the booster and master cylinder off to get the steering box out.

It looks like it had a manual brake master cylinder on it with the deep pocket for the pushrod. The push rod was too short for the pocket, so it wasn’t doing anything. The booster ran was contacting the MC piston to operate the brakes.

I called SSBC and they recommended a corvette power brake master cylinder with a shallow piston that would work with the booster correctly. They seem to know their business, but my question is about the residual pressure valve.

Since the corvette MC is made for front and rear disc brakes, it doesn’t have a residual pressure valve. Do I need one? From what I have read, they were eliminated in all modern cars with rear drum brakes.