Single vs. Dual Reservoir Master Cylinders


I have heard the argument many times, claiming a properly maintained single reservoir master cylinder is just as safe as a dual reservoir system. My response is always the same: “If a single reservoir system is so good, then why are they no longer offered?”

Often when things go wrong with a car mechanically, there is no warning they are about to fail, and most long-time enthusiasts have had their share of new or rebuilt parts fail as well. Brake systems are composed of numerous parts all working together to stop the vehicle. As cars evolved from four-wheel drum systems to disc/drum combinations, the design of the various components making up the system changed in order to keep pace with the new technology. One important change was the shift from single reservoir master cylinders to dual reservoir master cylinders.

Originally developed to handle the different fluid volume requirements for disc/drum brake combinations, it became quickly evident the dual reservoir systems also offered important safety benefits that were equally as applicable to the drum/drum brake cars as well. Single reservoir master cylinders provide pressure to both the front and rear systems; however, should a failure occur somewhere in the system, there is a very good chance that all brakes will be lost in the vehicle. With a dual reservoir system, the brake circuits are split into front and rear, and in the event of a failure, you have a much better chance of safely stopping the vehicle.

Typically in disc/drum dual reservoir master cylinders, one of the reservoirs is larger than the other. Often the larger reservoir is for the disc brakes, although some models of cars exist where this is reversed. Many new cars today have a single reservoir, although the reservoir splits internally when the fluid drops to a lower level. These systems also still use separate front and back brake circuits.

When using a dual reservoir master with disc brakes on the front, the output pressure is equal on each port and must be regulated or adjusted through an external proportioning valve to provide proper balance.

I understand the purist’s point of view, and although I strongly advocate for converting cars to dual reservoir master cylinders, at the very least, any enthusiast running a single reservoir master cylinder should ensure their emergency brake system is in top working order and test it often.

Share tips, start a discussion or ask one of our experts or other students a question.

Make a comment:
characters remaining

30 Responses to “Single vs. Dual Reservoir Master Cylinders”

  1. Gary Todd

    What bore size master cyl. do you recommend for a 67 Mustang with 4 wheel Aerospace Disc?

  2. Bradley Beran

    about 5 years ago I put radials on my 1951 Chevy pickup and converted to a dual master on 4 drums, two years ago I converted to a disk/drum set up and changed the master cylinder. the safety and, ride, handling and braking power improved greatly and I will never go back to bias ply tires and a 4 wheel drum set up. My wife just bought a 1951 Willys pickup, I have already ordered the front disk kit and dual master. Unless you don't drive your classic much and keep the braking system impeccable, retaining a single master and 4 drums is just not safe.

  3. Paul S Stanley

    I had an interesting experience on my 2002 Dakota. The two lines from the master cylinder were rubbing together after the plastic spacer must have succumbed to age and fallen off. The lines both burst at the same time. Fortunately I was on my property and in 4 wheel drive low, but still on a steep hill. The low gearing made it possible for me to have control and stop. I have a 1948 Chevy two ton that I was thinking of converting to dual, but decided not to. Getting the dual to work properly with the original linkage and balancing the pressure may have been more hazardous than the single system.

  4. Janedon Don

    I've driven a Lot of older cars (In rough condition) even just to get them home or to a friends place to work on--- Having driven a lot of older cars (in rough shape) I've experienced brake failure--Total failure in single reservoir systems (Scary) (I've even crashed) But with a dual system I've always maintained control--Redundancy is a good thing--

  5. Kurt

    I have a 1959 Edsel Corsair with drum/drum and the Treadlevac booster. Is there a conversion kit for my car? I also have a 1966 Plymouth Satellite that I would like to convert also. Is there a kit for that car? It is also drum/drum. Thanks!

  6. Robert Wingerter

    Owned 3 pre-1967 corvettes... each once, the first upgrade was to update to a 1967 dual MC. I learned the hard way in my high school car, a 1965 Impala Convertible. The brake line split in traffic and it was terrifying. Getting home was no joy either (backroads & emergency brake). Every old car since then, you guessed it. If you have a judged trailer queen - fine, get your show points, otherwise, it should be considered a mandatory upgrade to protect the car and yourself (in that order!)

  7. Bill Lopez

    I changed my single to dual on my 65 Impala with original drum brakes 👍

  8. Richard M Summers

    I have a 1932 Chevrolet Confederate, 2 door convertible coupe, old school HotRod. It has front disc, rear drum and a dual Master cylinder, The MC is mounted to the frame under the body, with no power booster. The brakes work fine but the pedal is kinda hard. Harder than other OE equipped manual brake cars that I have driven over the years. The firewall is clean for the no hood/hotrod look. All components are hidden. The MC has barely enough room to take the cap off and no service/inspection cover in the floorboard. No way to install the large drum style booster. How can I add an electric booster? Would I need a different MCyl because I see no fitting to accept a vaccum line? Do they mount to the MC or remote? Would adding an electric booster dramatically decrease my hard pedal? I want to keep the "Hidden look" and mount everything under the car. Like I said, The brakes work OK but the pedal is a little harder than I prefer. Also there is No parking/emergency brake at all. What are your suggestions? I enjoy and learn from your articles. Thanks. R.S.

  9. james d williams and gale k williams

    been doing this for many years. lost brakes on a single sys once the emer brake is almost no good. so a dual sys is the way to go. like to keep thing org but stoping is nicer. jim

  10. Bobby Phillips

    1957 Thunderbird with drum/drum brakes. Thinking of going to power brake system, and am reading conflicting remarks as to the need for a proportioning valve in the system. Needed or not?