I am changing from drums to factory 70 disc brakes. Do I need a different proportioning valve or the adjustable one you put in the rear line? I have a disc brake master cylinder. thanks

You need a disc/drum combination valve -


http://www.mpbrakes.com/technical-support/reference-valves.cfm


Reference/FAQs: Valves

What does a proportioning valve do?
A proportioning valve is used in the rear to decrease the rate of pressure rise to the drums relative to the pedal force as weight is shifted to the front during braking. This prevents the rear from locking up under hard braking conditions.

What are the symptoms of a bad proportioning valve?
If your prop valve has gone bad your rear wheels will lock up easily, especially on wet surfaces.

What does a metering valve do?
A metering valve or "hold off valve" is used in the disc portion of a disc/drum system to hold off the application of the front discs slightly allowing the slower reacting rears to catch up. This provides rear stability on wet surfaces and reduces excessive pad wear.

What are the symptoms of a bad metering valve?
Your car will nose dive and the front pads will wear too fast.

What does a residual valve do?
There are two different uses for residual valves. The 10 lb valve is used to hold a residual pressure to the drum brakes to keep the shoes out close to the drums giving a higher firmer pedal. The 2 lb valve is used in the disc when the master cylinder is lower than the calipers to prevent back siphoning of the fluid from the master.

What are the symptoms of a bad residual valve?
The brakes will be very spongy and you will need to pump the pedal to get good brakes.

What is a combination valve?
A combination valve incorporates metering and proportioning into one valve providing all necessary valving for disc/drum systems.
See below

Should I use an adjustable proportioning valve?
Not if you can help it. The adjustable valve will only provide the proportioning function and not the metering that is needed.

Descriptions


METERING
The metering or hold off valve is used in the brake system to better balance the front to rear brakes. The valve does not allow the pressure to rise at the front disc brakes until the pressure at the rear drums has risen sufficiently to overcome the brake shoe springs. At this point the valve opens to allow full pressure to build at the front brakes.

PROPORTIONING
The proportioning valve modulates the pressure to the rear brakes. The modulation is necessary to minimize rear wheel lock up found in heavy braking and to compensate for the differences in braking conditions in front disc / rear drum systems. As pressure is applied to the system full pressure is allowed to the rear drums up to a certain point. Beyond that point the pressure to the rear is reduced preventing rear brake lock up.

RESIDUAL
There are two different residual valves. A ten pound residual valve will maintain a line pressure to the rear to keep the drum brake shoes out close to the drums giving a higher firmer pedal. Without a ten pound residual pressure to the rear you will experience a spongy pedal. A two pound residual valve is required whenever the master cylinder is lower than the calipers to prevent backflow of fluid from the calipers to the master.

COMBINATION
A combination valve incorporates metering and proportioning into one valve. These are available for disc/drum or disc/disc systems.

ADJUSTABLE PROP VALVE
The adjustable proportioning valve is used when you have a special rear condition that requires higher or lower pressure than a normal condition. You sould always use a metering valve to the front when using the adjustable.

When I upgraded my fronts to disc they told me to try it without the adjustable proportioning valve as some cars operate just fine with the original one.

My car works fine with the original drum brake prop valve.