Ask Lugnut4x4: Can I Use A Drum Brake Master Cylinder with Disc Brakes?

Before converting to disc brakes, many people have asked us if they can keep their factory drum brake master cylinder. In fact, it’s a great question, and one of the most common questions we get from our customers.

Brake kit

If you’re buying a disc brake conversion kit from LugNut4x4, the answer is usually “YES.” There are a couple of situations where you may need to upgrade the master cylinder. We’ll outline those below. But in general, if it’s one of our kits, it will work just fine with the original brake system.

We’ve Developed Our Disc Brake Conversion Kits To Work With Original Equipment

Dana axle

At Lugnut4x4, we believe in using original parts whenever we can. So, all our disc brake conversion kits are designed to work with the OE brake system components. We have disc brake conversion kits for the following rear axles:

And all of these kits are designed to work with the original master cylinder.

There are a couple of situations where we recommend replacing the factory master cylinder, but they are not common. Specifically:

  1. The existing master cylinder leaks and/or doesn’t function properly.
  2. If you’ve got 8-lug (1 ton) axles under a Jeep or other smaller vehicle, the original master cylinder was sized for much smaller brakes, and will need to be replaced.
  3. If the front end originally had drum brakes, we recommend using a larger master cylinder with at least a one inch bore.

Typically, our kits are purchased with the standard JB6 calipers or e-brake calipers, and they’re installed on vehicles have the original master cylinder. So, typically, there’s no reason to worry about replacing the master cylinder.

What About Vehicles With ABS?

If you’re swapping out the drum brakes on a vehicle with ABS, our disc brake conversion kits will work just fine with the OE master cylinder and ABS actuator. However, we have an important installation tip: If you have a vehicle with ABS, do not let the master cylinder run completely out of fluid. If this happens, you’ll have to bleed the ABS actuator. Some are easy to bleed, and others require that a shop do it with their diagnostic scanner controlling the actuator while it is bled. This is particularly true with GM pickups and Suburbans from the 90’s. It’s much simpler to be careful and make sure the master cylinder doesn’t swallow some air.

What About The Proportioning Valve and/or Residual Pressure Valve?


Our disc brake conversion kits are designed to work with the original proportioning and residual pressure valve (or combination valve). We’ve spec’d our calipers to “balance” with the drum brakes they replace, meaning they’ll work best if the rest of the original factory brake system is unchanged.

So, other than installing our kit and replacing the brake fluid, there’s nothing else to worry about. Any changes you make to the brake system probably will not improve performance, and may in fact reduce it.

What If I’m Not Buying A Conversion Kit From Lugnut4x4?

If you’re buying a kit for a vehicle that we don’t have parts for, you definitely want to ask the kit manufacturer if they’ve designed around the original factory brake system. If not, they should be able to provide you with specs and/or part numbers for the components you need to install.

Generally speaking, most drum-to-disc conversion kits are designed to be compatible with the factory brake components. Still, it’s always a good idea to verify that before purchasing.