Auto Repair Help

by Kyle McFadden

All passenger cars and light trucks in the United States today are equipped with a dual master cylinder. The dual master cylinder, also known as a tandem master cylinder, is essentially two separate master cylinders combined into one unit. The dual master cylinder is used as a safety device, to divide the brake hydraulic system into two individual hydraulic circuits. There are two types of split hydraulic systems used. Most rear wheel drive vehicles use a conventional split system, dividing the brake circuits by axle. One half of the master cylinder is used to operate the front brakes, while the other is used to operate the rear brakes. A large number of front wheel drive vehiclesMaster Cylinder use a split diagonal system. The split diagonal system divides the brake circuits between one front wheel and one rear wheel on opposite sides of the vehicle. In the event of pressure loss in one circuit, the other hydraulic circuit would remain operational. The dual master cylinder has been required equipment by law, since 1967. Some automobiles manufactured before 1967 were required to be retrofitted by a certified auto repair mechanic for a dual master cylinder.

The master cylinder is generally constructed of aluminum or cast iron. Attached to or contained in the master cylinder body itself, are two separate brake fluid reservoirs. Master cylinders used with disc/drum combination brake systems will use a large reservoir for the disc brake portion of the system and a small reservoir for the drum brake portion. This is to allow for fluid level changes as the disc pads wear. Inside the master cylinder is a machined bore that contains the master cylinder primary and secondary pistons. Both pistons are combined into a single, two part unit. The section that is closest to the brake push rod is the primary piston, while the section closest to the front of the master cylinder is the secondary piston. Cup seals are placed on the primary and secondary pistons to provide a tight seal in the automobile master cylinder bore. Calibrated springs are placed between the primary and secondary piston and the secondary piston and the end of the master cylinder bore. The area in front of each piston is the pressure chamber area and this is where pressure is created to operate the brake system. There are two ports machined into the master cylinder bore for each brake fluid reservoir. One port is the by-pass port, the other is a compensating port. The by-pass port supplies fluid to the master cylinder bore when the brakes are not applied. The compensating port is used to allow the master cylinder piston to quickly return to the unapplied position, as well as compensate for fluid expansion or contraction from changes in temperature.

The master cylinder is operated by a push rod attached to the brake pedal. When the driver depresses the brake pedal to stop the vehicle, the brake pushrod moves the master cylinder primary piston forward in the master cylinder bore. The secondary piston moves at the same time, operated by hydraulic pressure created in front of the primary piston and the calibrated spring placed between the pistons. As the piston moves forward, the by-pass port is blocked by the cup seal, creating a sealed pressure area in front of the piston. Hydraulic pressure is created by the pressure of the piston on the brake fluid. When the brake pedal is released, it is possible for a vacuum to be created behind the piston as it returns to rest position. To prevent this, the compensating port allows fluid to flow into the area behind the brake piston.Master Cylinder Diagram

In the event of the loss of one hydraulic circuit, the remaining circuit would be operated in part by the failed circuit piston and the brake pushrod. If the primary circuit were to fail, the loss of pressure would cause the primary piston to move forward in the master cylinder bore, until it bottomed against the calibrated spring and the secondary piston. The primary piston would then act as a connector between the brake pushrod and the secondary piston. If the secondary circuit failed, the secondary piston would move forward against the secondary piston spring, until it reached the end of the master cylinder bore. When the secondary piston reaches the limit of its travel, the primary piston is then able to build pressure in the primary brake circuit. Of course, when diagnosing and troubleshooting the automobile master cylinder and overall braking system, both hydraulic circuits must be tested by an auto repair mechanic for 100% functionality.

Most master cylinder malfunctions are the result of failed seals, as any auto repair mechanic will attest to. Leaks can develop at the rear seal of the master cylinder, resulting in fluid loss at the pushrod area. Cup piston seals can harden or wear, causing brake fluid to leak past the brake pistons, resulting in loss of pressure inside the master cylinder. Poor internal sealing due to worn or hardened cup seals is most evident by the brake pedal slowly sinking to the floor during periods of light brake applications, such as sitting at a stop. If you are not a do-it-yourselfer, make sure you have an auto repair shop inspect your master cylider at least once per year. Generally, the best auto repair facility to have repairs performed is the dealer, although the dealer is usually the most expensive. Independent auto repair shops can be just as reliable, however. Check with your local Chamber Of Commerce or Better Business Bureau to find a reputable auto repair facility.

(Kyle has an affinity for Pale Ale and tooling on his 1956 Chevrolet Nomad Station Wagon.)

22 responses to “Master Cylinder”

  1. Ira says:

    I’m putting together a presentation on master cylinders. Would I be able to use your drawing contained in this article for the presentation. It demonstrates the transfer porting extremely well.



  2. beverly glass says:


  3. Chris says:

    Do most master cylinders share the exact same seal sizes?

  4. Mark W says:

    “Master cylinders used with disc/drum combination brake systems will use a large reservoir for the disc brake portion of the system and a small reservoir for the drum brake portion. This is to allow for fluid level changes as the disc pads wear.”

    What about if you have four drum brakes on an old van? Will it matter if the master cylinder has two different size chambers? Which chamber should go to the rear and the front?

    • Pem says:

      Rear should usually be the larger of chambers in that case, since the fluid has to fill more line to get there.

  5. Jay says:

    Hey there I have a 77 Harley flh I had a problem where my calliper held on to the rotor causing the bike to slow then stop I waited a hour then it let go when cooled! I avoided using them! I went home pulled the calliper then rebuilt it put it on now the master will not build psi? I wonder if there a by pass or some way of resetting the master? Could it somehow have broke the seals when trying to pump it up? What should I do?

    • Jeff R says:

      you should try vacuum bleeding the brake system this will usually work depending on the condition of your master cylinder. You may want to replace your rear brake hose also, they can develop flaps and blisters inside that prevent the fluid from returning to the master cylinder causing the brake to remain applied

    • Hyzoot says:

      I am afraid the “Forrest for the trees” applies here. It sounds like your “floating caliper” is not floating.
      The heat causing the drag on brakes might indicate the bracket clearance needs to be checked, or the “slides” need greasing.

      This “sticking slides syndrome” could present as a “no pressure” issue during bleeding.

      I hope this helps

  6. Nelson H. says:

    on a master cylinder with 2 output ports, are both ports supposed to produce the same amount of pressure going out to each circuit?

  7. olarewaju says:

    What can cause the vibrating and limping of toyota carmry break when it is march?

  8. Virginia says:

    Have a 1995 GMC van which one day I got in and started out and realized I had very little brakes. So I immediately parked and then checked the brake fluid and found the small cup was completely empty. filled it up to see I could find a leak, let it set a 2 weeks and tried it every 2 days and kept a check on the fluid. Did not go down and had plenty of brake pedal. Took to shop to have it checked and they found no problem. Do you have any idea where the brake fluid could have gone? Should I bee concerned?

  9. […] But if you think about when you are bleeding your brakes, the pedal still hits the floor whenever a bleed screw is cracked open, either at the front of rear..? So how does the pressure build up enough, to actually activate the brake system that is still intact? Interesting read.. Master Cylinder – Auto Repair Help […]

  10. Brandy says:

    I have a 2000 yukon dinali 5.7 abs. My father replaced the master cylinder twice.The pedal drifts to the floor.He feels the brakes grabing when just taping the pedal,then,the pedal drifts to the floor.we checked for leaks,none . we blead brakes twice,two ways. first way was one person way,with jar and hose to do a flush. second way was pump and open bleader. good stiff brake,than drift to floor. power boost is fine,foot on pedal start truck and pedal moves down to point of having brakes,than drifts to floor.

    • Johan says:

      I have the exact same problem with my 1985 Mercury Colony Park as your Yukon. Did you ever solve it?
      if so, please! How….?

  11. Frank Lampone says:

    I need to replace the switch cover on the drivers front seat of my 2008. GMC ENVOY need help how to remove the buttons on the switches Thank you.

  12. Steve Kline. says:

    looking for the cup seal 7mm i.d…. and secondary seal i.d.9mm for the rear master cylinder..5/8 bore from my 1980KZ 1000 G model ..seemingly impossible to find… no replacements or rebuild kits are made for this unicorn..are they available still? or do i need to fab up some other cylinder? grabbin at straws here thanks for any help or info you can give

  13. wayne says:

    i have a power boosted dual master cylinder brought for a front disc brake conversion but now have score some rear disc’s can i still use this master cylinder .Car is 66 drum brake mustang

  14. matt says:

    I have a 1972 ford galaxie500 and I was wanted to know on the master cylinder which on is front brakes and which is back

  15. Josh says:

    No brakes, peddle go’s all the way down. Master cylinder leaking at connection to brake booster. Drum rear brakes, discs front. Fluid levels= my secondary is full and my primary was empty but since my secondary still was full shouldn’t I still have some brakes? So does that mean not only do I have the leak to deal with but also a problem with my front disc brakes that I haven’t noticed until now?

  16. I have a 97 Mercury villager and the caliper on my driver side is locking.. I was told that it was the master cylinder cup piston , but I don’t know.. I have changed the caliper to a brand new one and even got a remanufactured master cylinder which did not come with a cup piston… I need some advice or help
    .. what can I be..I try to push the caliper back in after it locks up but it won’t go in, now when I open up the brake line on my master cylinder,it goes back.. what could it be

  17. Ann says:

    What could cause my reservoir to go dry in the front but over fill the back one?

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