From marlins site>>>
"Replacement master cylinder for older Pickup's and 4Runners with smaller bore (bore size is cast into right side of housing). Fits 1979-1995 Pickup/4Runner. Features 1" piston and residual valves for both front and rear. Works with both drum and disc brakes. Bolt on installation. Includes reservoir. On 1979-1985 Trucks and 4Runners the smaller factory bore works fine with stock, solid front rotors and 8" rear drums. If you upgrade the rear to larger 1986 and up, 10" drums and/or upgrade the front to a vented style caliper you will want to install a 1" bore master cylinder like this one for additional volume.
Upgrading your brakes without changing the master cylinder results in excess brake pedal travel. If you have upgraded your brakes on a 1979 to 1985 truck and now you have to push the peddle nearly to the floor, this master cylinder will make the peddle feel like it should."
