Do I need a proportioning valve?

May 1, 2009 | By Richard Prince


I want to change my single-reservoir master cylinder to a dual-reservoir unit as you advised the 1962 Mercury owner in the September issue. It would be neater to have the lines come out of the right side rather than the left.

My rear axle is bad, too, and I have a replacement from a parts car that is a heavy duty unit with 11-inch drums instead of my original 9-inch drums.

A parts man here said that I will need a proportioning valve but he couldn’t supply one. Can you suggest a specific cylinder that will work and do I need to worry about a valve?


In general, you want a master cylinder designed to work with the setup you have, which is either front disks and rear drums or four-wheel drums. Contact Master Power Brakes in Mooresville, North Carolina, for a more specific recommendation (

The primary purpose of a proportioning valve is to modulate pressure to the rear wheels so as to avoid rear wheel lockup during heavy braking.

A proportioning valve is normally used only in front disk/rear drum systems so if this is what you have then, yes, a proportioning valve is necessary.

If, however, you have drum brakes all around then you should not need a proportioning valve.