720 Brake Master Cylinder Ports

[Barn 720]

Hi everyone,

 

I'm trying to replace my original Nabco 7/8 brake master cylinder for my 1981 720 4x4 (May 1981 build date). I bought a new replacement from Centric (13042306), but noticed the front and rear ports are in the opposite position from the original. Otherwise the Centric looks fairly similar to the Nabco unit. I know from the factory there were also Tokico master cylinders, and perhaps there were changes to the brake system for the 1982 model year (I know they added self-adjusting brakes, so maybe there were other changes as well?).

 

Does anyone have any insight into the variations of the master cylinders of this era and possible differences between cars and trucks? 

 

Do the front and rear systems operate with different pressures? If I were to use this MC, would I necessarily need to reroute the hardlines to the new F and R positions? Or would I be better off locating a new MC with the correct ports? 

 

Thanks for your help.
 

 

 

 

 

 

[ggzilla]

Tokico and NABCO units are functionally identical for the same application. Individual parts can differ like the rebuild kit seals, the but the masters (as far as I've seen) are interchangeable. Perhaps they sent the wrong year part? It should have F on the front of the master

 

front and rear brake have the same pressures. The pressure is generated by the Master Cylinder, and it is the same size bore for both front and rear

8 hours ago, Barn 720 said:

Do the front and rear systems operate with different pressures?

 

The F and R ports on the master cylinder are different because the check valves differ and the reservoirs differ. But on the 1981 4x4, it is a shared reservoir, so no worries there. I don't know if you could just swap the check valves?

 

RockAuto sometimes gets their parts mixed up, and sometimes the supplier does. I see they list two different incompatible ones for 1981 4x4, both listed as interchanging with Nissan 46010-48W00 but have the F and R reversed. I would tell them which one you need (for example, F front outlet, R outlet in middle) and see if they will exchange it for the one you need

[Barn 720]

Thank you for your help, ggzilla. When I search the aftermarket brake master cylinders I find a lot of variation as well; most with the reversed F & R ports like the Centric, some unlabeled, and only a few with the ports in the correct order. Even though most listings state for 80-83 720 only, I wonder if there were some other vehicles that these aftermarket units are intended be used with (or at least used the same the MC housing). It seems the more common set up for most vehicles is R in front and F in rear, maybe just another way that Nissan is unusual.

 

I can't be the only one who has wondered about this, given the variety of MCs on rockauto, oriellys, autozone, napa, etc. Maybe some people just don't notice and hook it up in the same configuration as original and don't even worry about F & R. I guess you'd know you have a problem if the rear brakes start locking up.

 

That said, I've decided to go the sensible route and return the Centric and I located a NOS NABCO unit. Better safe than sorry...

[datzenmike]

Have never ever seen a Datsun or Nissan master cylinder with the F to the front.

 

The front and rear pistons in the master are not physically connected and separated by a spring. As the front brakes do more braking than the rears it's closest the peddle input. The first to move is the F (at the rear) applying brake line pressure to the calipers and at some point the piston travel compresses the spring and begins to move the rear piston. I would say the front line pressure is higher than the rear system? 

 

How the F and R reversed works is beyond me. 

[Barn 720]

Thanks for the reply, Mike. That's what I would assume too -that the valve closest to the peddle input would engage first, so it makes logical sense for the F system to be located there. I wonder if Nissan was attempting some sort of innovation by switching the ports in the 720. As ggzilla said, maybe the difference between the two ports has to do with the location of check valves that keep some residual pressure on the R drums. 

[ggzilla]

Nah, this is a hydraulic system. The port arrangment has nothing to do with location. A master cykinder can have F at front, middle, rear or bottom

[datzenmike]

Been studying the master and I can see where the F and R circuit pressures are likely the same. The deciding factor is the F and R can be reversed though I have never seen one on a Datsun.