Rear Master Cylinder froze/replacement

[Jamps]

Hey,

So I've been working on getting my "new" 79 xs400 onto the road, and the problem I'm facing is the piston in the rear master cylinder is stuck inside the cylinder.

I bought a rebuild kit for it that replaces the boot, seals and the spring, but that isn't really going to help if I can't get the piston out.

And even if I get it out i know it is possible that the cylinder might be pitted and too gnarly to even use.

So, my question if anyone can help is, is there a way to get the piston out? and if not or if I do and the cylinder is unusable does anyone know of a good replacement rear master cylinder for the 79 Xs400?

Thanks,
Ryan

 

[JPaganel]

I wrote this big thing thinking it was about the caliper...

Does the piston move at all? Or does it get to some point and stick there?

 

[Jamps]

I wrote this big thing thinking it was about the caliper...

Does the piston move at all? Or does it get to some point and stick there?

I mean, even if it was about the caliper it was super informative so thanks for that, I copied it down into my notes.

The piston in the caliper was actually also stuck though not as bad. I used those piston pliers tool and with a little bit of elbow grease I was able to get it out.

On the other hand, The piston in the rear master cylinder doesn't move at all. It is frozen stuck and I have no idea how to break it free, or if it is even possible to do so.

 

[Jamps]

Also, does anyone know the diameter of the rear master cylinder on the 79 xs400?

 

[JPaganel]

The piston in the rear master cylinder doesn't move at all. It is frozen stuck and I have no idea how to break it free, or if it is even possible to do so.

It's unlikely to be stuck on the inside. Corrosion is more common outside.

Is there a whole bunch of crud around the pushrod?
Did you manage to get the clip out?

You can try soaking it in penetrating oil.
You can also try soaking the end in CLR (that's the bathtub cleaning stuff). It's pretty good a softening up corrosion.

 

[amc49]

I would not use CLR, it may corrode even further. And all brake fluid unless synthetic is hygroscopic, meaning it absorbs water and corrosion will be worse on the outside but can easily be past the piston too.It almost always settles in right where the piston sits.

Soak in lite oil or WD40 like said for several days and while doing so, cobble up a fitting that goes into the master and at the other end hooks up to a common grease gun, then pump the piston out. 3000-3500 psi will move it or break the casting open.

Hone the cylinder body out and if still corroded right where the piston sits it is no good. You can take a 1/4" bolt to cut the head off or use a stud and with a saw cut a 3/4" length slot in the end to hold a folded up piece of sandpaper and the best cylinder honing tool ever when combined with a drill motor. I use like #400 paper in mine.

 

[Jamps]

UPDATE: The rear break now functions perfectly! Piston got removed using Blaster, the part was fine, no pitting or any damage.