Jul 27, 2026

How Do I Convert a Lockheed or Wagner-type Rear Master Cylinder From Drum To Disc?

by Liam Irving

Putting a new master on a Pan or Shovel? You might be aware there's some trickiness in navigating the master cylinders.

When Harley-Davidson switched from drum to disc on the rear of the Big Twins in 1973, the rear master cylinder has the same external appearance as the earlier models, but the internal components are different. You need to set yours up correctly before installing.

Drums use a residual pressure valve. Effectively, this valve leaves some hydraulic pressure in the line. Since shoes have a spring that retracts them from the drum, the residual pressure this valve leaves in the line keeps the shoes right up on the drum so you don't have to two-pump your pedal to get some brakes. Discs, having no such spring, will apply pressure to the caliper pistons if a drum-type master cylinder is used.

If you see a master listed for 1958-E1979 fitments on our site, you should know that the master will indeed work for any year listed. If you have a rear disc, you must remove two pieces to make this master function correctly on your bike. Here's a disassembled image of how we ship these master cylinders:

See that arrow? The two parts, the spring cup and rubber-coated disc, make up the residual pressure valve. Pull those out, set them aside, and reassemble.

When you're done, you'll have something that looks like this:

Note that the pieces in the images are laid out in the proper orientation with the exception of the rubber cup at the narrow end of the springs. On that item, the wide end goes to the front of the bike when installed.

If the RPV is in place and the master is used on a disc brake, the brake will be "sticky" since the shoes aren't retracting enough. If the RPV isn't in place and it's used on a drum, you'll have a crummy pedal that takes a few pumps to build pressure. Once they're set up correctly, you can bleed as normal, and go ride confidently and safely!