This is discussed on All-Pro's site:
FJ-80 Master Cylinder
The stock Pickup and 4Runner master cylinder on 1979 - 1995 rigs are designed for discs brakes up front and drums in the rear. It does not have a residual valve for use with rear disc brakes. Without a rear residual valve in the system the rear disc pads expand after braking. This causes excess peddle travel the next time you step on the brakes. To solve this we sell a FJ80 brake master cylinder that is equipped with two residual valves, one for the front system and one for the rear. It's 1" bore size is larger than older 13/16" bore cylinders for more fluid volume.
Aside from the misspelling of "pedal" (and the fact that the pads don't 'expand', they move away from the wobbly rotor as it pushes them away), there is some disagreement on their assertion that minitruck MCs cannot control rear disks due to runout. It is possible that with a non-FF axle, runout can get pretty severe; I know Brian Ellinger's kit with dual tapered roller bearings is much less likely to have this issue. Brian disputes the residual valve assertion above - and I know lots of folks run minitruck MCs with few issues.
However, since the FJ80 MC also has a 1" bore, and we run large tires, and typically IFS calipers on the front, and front calipers on the rear (lots of fluid volume in other words), there are lots more reasons to use a 1" bore MC. It works really really well when properly matched to a vacuum booster from a V6 minitruck, or I hear a Tundra or such.
FWIW, I am running a 1" no-name ( <img src="/forums/images/graemlins/shame.gif" alt="" /> ) MC for a Tacoma I believe, and it controls the rear disks ('85 4x4 front calipers, woo hoo!) just fine. But I am also running Brian's FF kit.
'SUDDEN' would be a good word to describe how the brakes work. As 'SUDDEN' as you wanna be, with the prop valve set as you wish. Towing, etc. is no problem.
