Brakes 66 mustang disk brake conversion

My proportioning valve (66 coupe, front disc conversion) is in-line on the rear brake line. It is not hooked to, or part of, the master cylinder. Disc and drum take different pressure. The valve lets you adjust the pressure to the rear. If I did attached the picture right, you should see the prop. valve down under the master cylinder with the front bowl line running through it on the way to the rear brakes.

There is no proportioning valve on factory 4 wheel drum brakes. It is needed when front disc brakes are installed, otherwise the rear brakes will lock up too soon. This can cause the rear of the car to pass the front in a panic stop.

In addition to adding a proportioning valve for the rear drums, you will also need to either swap to a disk brake specific MC OR remove the residual valve from the MC port that will feed the discs. It is located behind the seat of the appropriate MC port, where the brake line screws in. Typically the rear port feeds the discs and its reservoir should be noticeably larger in capacity.

If your dual reservoir MC does not have the larger reservoir at the rear, then I would certainly purchase a different MC made for disc brakes. Disk generally will require more fluid in the system when the pads wear. I used a dual reservoir MC from a 78' Maverick with manual disc front brakes and drum rears. It bolts on the firewall the same as the one you have now and may need some line adapters to mate your existing lines.

A photo or 2 of your MC and distribution block would be beneficial for recommendations on how you should proceed.

I just got a MC for a 70 mustang disc/drum. Bolts right up and works great. You will still need a proportioning valve. You plum it in just like a line lock in the rear brake line. They run about $40 bucks. You can use your current mc but as Dennis112 mentioned, you will have to remove any residual valve behind the line seat on the front brakes only. Depends how old the MC is as most rebuilt and new parts store MC no longer put them in. The residual valve holds 10 lb of line pressure on drums. This will cause your disc brakes to drag all the time if not removed.