Brakes '86KC 4x4 - seal between master cyl and booster, also how many proportioning valves

[MaddieCycle]

I am getting ready to replace the master cylinder and notice on the parts fiche there does not seem to be a seal or o-ring between the vacuum booster and the master cyl body.  Just checking on this so I don't pull it all apart and then find out I need a seal that is NLA.

 

Also, verifying that there is only 1 proportioning valve in this truck, passenger side under the bed, and there are no other valves in the system to service.  Does anyone ever replace the prop valve as part of routine maintenance? Assuming it is isolated to the rear circuit so even if it failed you'd still retain your front brakes in an emergency.....  I don't have any visible leaks in the system besides the master cyl, so I was only planning on replacing the hoses and not the calipers or wheel cylinders.

Thanks!

[datzenmike]

I don't think there is anything to seal they just bolt together.

 

That's not a proportioning valve. That's the NLSV Nissan load sensing valve. You'll note that it is slightly tilted up at the front? (10⁰) There is a steel ball and spring inside that allows brake fluid pressure to the rear. As cargo weight in the bed lowers the rear of the truck, the ramp becomes steeper and the moving ball cannot seal effectively and more brake pressure is allowed to the rear tires that have more weight on them and thus less likely to lock up.

 

There are two residual valves in the master cylinder. One each for the front and rear circuits. The residual valves retain a slight pressure in the system... about 2 PSI for disc brakes to chafe the pads lightly against the rotors to keep them clean. The rears are much higher at around 6 PSI for drum brakes that have farther to travel before contact. These must NOT be swapped by mistake.

 

None of these brake parts need replacing as any part of a routine maintenance. They are replaced as they fail which is basically.... never.

[MaddieCycle]

Thanks this is super helpful.  What happens when the truck is on an uphill? Doesn't that also mess with the load sensor?

[datzenmike]

Going up hill it would apply harder but I imagine there would be less weight transfer off the rear wheels so it averages out. Down hill more weight transfer and less braking pressure so that averages out too.

 

Just my opinion. Obviously it works or doesn't seem to harm anything.