Master Cylinder leaking

[Slartibartfast]

I cleaned the cylinder and tore it most of the way apart tonight.

 

Looks like it was OEM under all that gunk.

 

Interesting machine work on the mounting tabs... I didn't expect to find anything ornamental on a brake cylinder.

 

Random stuff:

The sensor for low brake fluid is a magnetic reed switch wedged under the reservoir. The float (some kind of hard closed-cell foam) contains a magnet.

 

The FSM suggests removing the piston by shooting air in through one of the brake line connections. That didn't work for me, probably because the res was off already, but shooting air in through one of the holes between the res and the cylinder did the trick.

 

The piston looked fine, the bore looks like a mirror, and the stock bushings (the ones I tried replacing) were fine. In fact I can't easily tell the old bushings from the new ones. Oh well, they were cheap.

 

The very end of the cylinder has a big allen bolt hidden behind a thin piece of aluminum. There's clearly something behind that bolt, and I'm guessing it's a proportioning valve for the front vs rear brake lines (as identified by F and R on the casting?).

 

 

 

The mystery hole is unthreaded and shows no indication that it ever had or would accept a plug. It steps down into a smaller hole (0.5mm or so) and goes into the proportioning bit. I can't stick a wire more that a millimeter into the smaller hole, so there's clearly something in there. I haven't gotten into the chamber behind it since the steel bolt has seized to the aluminum housing. I plan to soak it with PB tonight and give it another go sometime tomorrow. I don't figure I'll put it back together (no core charge for a new part), but just for a change I figure I'll try and take it apart without a claw hammer or a hacksaw.

 

Oh and Rob, I don't know which aftermarket MCs you've dealt with, but mine came with a new res. I'm glad it did because the old one had a lot of crud in it.

Edited February 17, 2013 by Slartibartfast

[Trogdor636]

In my experience new aftermarket come with reservoir, remanufactured do not.

[Slartibartfast]

^^ Makes sense.

 

 

The whole non-destructive teardown went south about the time the allen bolt stripped. But it's apart, and the bit in there does contain a sprung valve (which showed some signs of wear). There are two little holes going from the main cylinder to the secondary, both drilled through from the outside. One comes in at an angle through the hole at the end. The other looks like it came in through the mystery fitting, which (if my uncle's right) was stopped up with a little bearing after the fact. I'm not 100% on how the valve worked (partially because some of it came out wrapped around the drill bit) but my curiosity is satisfied... except now I wonder how the new master was made without that hole. Gotta be a different valve, or maybe they drilled from the main cylinder in... yeah, I'm probably putting too much thought into this.

 

Anyway, road test tomorrow.

[Slartibartfast]

Works fine, pedal's got a little more free travel than I remember but it still stops just fine. Either it'll settle in or I'll get used to it.

[Precise1]

Glad it fixed it fairly easily and cheaply.

 

Thanks for the pictures as well, they are always good to have.

 

B

[Slartibartfast]

So I go and look under the truck today (unrelated) and there's no oil on the bottom of it. For as long as I've had the thing it's had this slow leak that leaves little droplets of (what I assumed to be) motor oil on the bottom of the crossmember. I guess that was brake fluid this whole time because there's no oil there now!

[devonianwalk]

Sweet! I'm hoping that a slow drip coming from the back of my 3.3 and the tranny is just the driver side valve cover gasket (dangum plenum .. right?) and not the RMS.

[BBB]

Thanks for the OP and the pics of the tear down. I too have this same issue.

[Macrow]

Good Post was very help full as my 95 XE Pathfinder seems to have this same problem going on which started on me last night

[dirtbike23]

same thing happened to my terrano's MC, replaced it with an aftermarket one with no mystery hole also, until now been wondering what the purpose of it. nice post.

[Slartibartfast]

Holy thread necro, Batman.

 

I just had the same failure on my '93, tore it down with the right tools, and got a better idea of what's going on in there. Looks like that little hole is actually some kind of air/fluid drain that lets the proportioning valve move freely. It was probably weeping fluid because the seals on the prop valve were shot.

 

I also opened up my new (garbage) MC that's got no bleed there. It uses a different type of prop valve with no external springs, just one big block with two O-rings and some holes that I assume lead to internal valving.