Brake master cylinder rebuild–I. Honing.

Thanks to my patient reader (notice the use of the singular noun there) for not removing The Green Truck from your blog roll, despite the lag in substance this spring.  If there are more than one of you out there, well, great.  Hopefully action on the blog will pick up again.  This is a good place to start.

I talked about removing and disassembling the brake master cylinder in an earlier post.  This one continues the process in two parts:  the first one on reconditioning the bore, and a second one on reassembling the master cylinder (and hoping it is in working order).

Green rarely had servicing and what was services used to be fixed with baling wire (and yes, there are still pieces to be seen holding this or that together).  I’ve mentioned that there are absolutely no brakes–not a drop of brake fluid to be found in the lines.   Brakes are high on the list of necessaries on an automobile, so obviously something needed to be done.  I’ve been trying to find time to do this for six weeks, but I came home an hour early on a Friday afternoon and press-ganged Son Four into acting as a photographer so that I could document the work.

The bore of a brake master cylinder is normally smooth.  As you can see (below), the cylinder walls from Green’s cylinder are hardly smooth any more.  What has happened is that over time a tiny bit of moisture in the line has pitted the steel sides. That rust creates depressions that allows brake fluid to leak past the primary seal.  Pushing on the brake pedal becomes less effective because the hydraulics are compromised and don’t transfer the force effectively to the brake cylinders in the wheels.


Pitting in the bore.

The brakes were failing as I drove Green in high school in the late 1970s and early 1980s. Thirty-five years of sitting in a field hasn’t helped.  To again function correctly the cylinder bore has to be smoothed out or replaced entirely.  Since I am trying to keep the original equipment on Green, the available options were fairly direct–the cylinder has to be honed smooth. Honing a cylinder is technologically simple but requires a specialized tool that would be used once and never again.  I still bought one.

A hone is nothing more than a trio of small grinding stones set on springs at the end of a shaft. Having three spring-loaded stones keeps them centered evenly against the bore walls as the shaft turns.

The challenge of honing a bore is to stay within tolerances:  just enough steel has to be removed to level the pits, but not so much that the primary and secondary seals no longer fit tightly.  After doing some exploring, the folks who have done this say that one aims at taking off no more than 0.004 of an inch, which enlarges the bore by twice that figure or 0.008 inches, because there is a bore wall on the opposite side as well.

Notice that the hone fits into a standard drill.  I could have maybe used a hand-operated eggbeater drill, but that would have been difficult to both hold and operate effectively.  As it was, I had to use both hands and compromised on the setup–I was supposed to use solvent to flush away the particulates from the grinding and prevent oxidation, but I followed DIY advice to use hot water, since I don’t have a shop solvent fountain.  Son Four did a thirty-second video of the hone in action, but my WordPress subscription does not allow me to post video files, so here are a couple of stills that show how it works.


After the bore was honed and while it was still wet, the cylinder went into the oven to dry it out quickly.  As it was there was still a film of oxidation (rust), which I wiped out first with a shop towel and then a bit of brake fluid to coat the steel and keep it from oxidizing again.


As you can see in comparing the two images, there are still a few shallow pits, but conventional wisdom is that something that small probably won’t affect the breaks much because the primary seal should be flexible enough to ride through them.

So, now all that needs to happen is to put everything back together.


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