Master Cylinder ( maybe leak? )

[windcalmer]

I was getting my 94 Pathfinder back up and running, replacing belts, etc. The brakes had a leak back when it was running, but I never had time to track it down. The leak was small and would need new fluid added every two weeks or so. So we bleed the brakes and everything works fine. We check all the connections, and hoses and nothing is wet. No wetness around any of the calipers either. Then I notice there is fluid leaking out of a hole on the master cylinder. (Attached photo shows were the hole is). So I go looking for a new master cylinder and notice that around half o f the photos I see have that hole totally covered up.

 

So my question is could a screw have been in that hole and fell out causing this slow leak? Would covering that hole prevent the leak?

 

[Slartibartfast]

I've had two OE masters fail the same way as yours. At first I assumed that hole was supposed to be plugged, but after inspecting the stripped-down carcass of the master from my '93, I think it's actually supposed to be open. I can shine a light right through that hole into the bore where the rear brake proportioning valve lives (behind the silver sticker and the big Allen plug at the end of the master). The manual doesn't show an exploded view of the prop, or explain how it works, but as near as I can tell, it's basically just a piston with a spring behind it softening the pressure to the rear circuit (giving the fluid another place to go). The air behind the piston has to go somewhere, and that somewhere is out that vent hole. This works fine until the seal on the piston starts to leak. Sealing the vent hole somehow would stop the leak, but it would also lock up the prop valve, so don't do that.

 

Rockauto has a piston kit for this master, but the picture shows the main pistons only. So that's no help. I'd be surprised if Nissan even showed the prop valve components in the parts diagrams given they're not in the manual, but it wouldn't hurt to check. Aftermarket cylinders have a different style of prop valve (I don't know how they work) and the boss for the vent hole isn't drilled. Maybe that same style is retrofitted into the reman ones, and that's why the hole is plugged off.

 

TL;DR your MC's whupped. Make sure you match the replacement MC to your rear brakes, drum or disk. The listing should say which system the internal prop valve is set up for.