Cleaning an old master cylinder

[The Veg]

This isn't actually from a BMW, but I figured I'd ask here since this section gets the most traffic.

I've got an old master cylinder that needs rebuilding but first I need to clean out a crust of dark brown oxidised brake fluid residue. What's everybody's favourite stuff for dissolving that mess?

[khittner1]

No personal experience, Veg, but I think one correct answer is probably "clean brake fluid". It may not be the most effective solvent, but any residue left from its use presumably won't mess with the rest of the brake system when you button it back together.

[George Ryals]

Since you are not concerned with harming o-rings and seals, I would think anything that dissolves stuff like brake fluid would work. Acetone, mineral spirits, "brake clean", carburetor cleaner, etc. Once the crude is gone rinse in hot soapy water to get rid of any solvent residue.

[The Veg]

Thanks guys. Neither mineral spirits nor penetrating oil gave much result, so I'm trying brake fluid now. Stuck a rolled-up paper towel into the bore and poured-in some fluid, set the thing at an angle that should prevent fluid coming out any of the holes, and I'm letting it soak a while.

[Ken in Oklahoma]

Thanks guys. Neither mineral spirits nor penetrating oil gave much result, so I'm trying brake fluid now. Stuck a rolled-up paper towel into the bore and poured-in some fluid, set the thing at an angle that should prevent fluid coming out any of the holes, and I'm letting it soak a while.

Hmmm, I have to wonder, Veg, why you're not trying some of that spray brake cleaner stuff. Almost, by definition, that would seem to be the stuff to use.

Ken

[The Veg]

Good question Ken! I guess it didn't occur to me due to limited experience with the stuff. The once or twice that I've used it it seemed like it evaporated in the blink of an eye, which made me think that it wouldn't do much vs. a hardened crust. I figured it was more for cleaning-up fresh fluid and dislodging brake-dust.

You're prob'ly guessing that this is a Lotus-project and you'd be right. The current Lotus came with a funky brake-system in which the master cylinder is installed with the open-end facing toward the front of the car and driven by a pushrod above the axis of the pedal hinge, then downstream from that, US models were fitted with a pair of vacuum-servos that were mounted on top of the left front wheel well. This is because a traditional single servo mounted to the master cylinder won't fit on the firewall. The servos on mine are shot, and are not rebuildable. Replacements from another manufacturer are available, but instead of spending all that money and keeping the system complicated, I'm eliminating the servos altogether, which necessitates switching to a smaller-diameter master cylinder from one of the UK-market cars that didn't have servos. New ones are not available, but I found and old used one that had the pistons stuck in it and a good buildup of the aforementioned crust. Rebuild kits for it are easily available, so once I get it de-crusted it'll get new seals and go into my car.

[The Veg]

Forgot to mention, this old Lotus is a lot like an airhead so it's all good. It uses a pair of Stromberg carbs that are basically bigger versions of the familiar Bings, the body is made of a GRP really similar to what airhead fairings are made of, and the electrical system is fairly simple (even if less reliable). The airhead I had could be mostly dis-assembled with a couple of allen wrenches and a few other tools; this Lotus can be mostly dis-assembled with a 7/16" spanner and a Philips screwdriver.

[Ken in Oklahoma]

. . . this Lotus can be mostly dis-assembled with a 7/16" spanner and a Philips screwdriver.

No stinkin' 11mm wrench for an Anglophile?

By the way, with my new dalliance with Corvettes I've learned that General Motors bought a huge chunk of Lotus. And it was Lotus expertise and technology that led to the ground shaking power (and durability) of the mighty C4 Corvette ZR1. I was impressed learning that, and somewhat abashed by my (former) mental image of Lotus as something like a quaint British manufacturer. I should have known better since Lotus has been such a power house (no pun intended) in racing.

Ken, abashed in Oklahoma

[Snakeoil]

Denatured alcohol is the brake solvent of choice.

Rob

[The Veg]

No stinkin' 11mm wrench for an Anglophile?

I have yet to find a metric fastener on this car. It was designed in the mid-'60s, as a development of a model that first came out in '62. I think the British auto-industry started using metric stuff much later. But hey, at least I also have not found any Whitworth fasteners on it.

By the way, with my new dalliance with Corvettes I've learned that General Motors bought a huge chunk of Lotus. And it was Lotus expertise and technology that led to the ground shaking power (and durability) of the mighty C4 Corvette ZR1.

Old news. GM acquired Lotus in '86 and sold it in '93. The LT5 engine was a good one, designed by Lotus and built by, of all companies, Mercury Marine. Lotus definitely benefited from GM too, as access to GM's global parts-bin allowed great leaps forward in product development, seen most obviously in improvements to the Esprit but also in such things as the M100 Elan, which was a flop at the time (as nobody wanted an Isuzu-based, front-wheel-drive Lotus) but really was a nice car that handled well and is now gaining status as a sought-after rarity. I have a friend who picked one up a year or two ago and he loves it!

I was impressed learning that, and somewhat abashed by my (former) mental image of Lotus as something like a quaint British manufacturer. I should have known better since Lotus has been such a power house (no pun intended) in racing.

And as a result of all that racing prowess, Lotus became a HUGE powerhouse in doing engineering under contract for other companies. This actually is where they make the most money, rather than by selling cars. Lotus engineering is so well-established and respected that pretty much any modern car you see in traffic has at least one Lotus patent somewhere in it.

The quaint old days more or less went out in the '70s or '80s, which is not to say that Lotus ever became a big, profitable company. The world had to be kept up with, but even little shoestring-companies, with enough determination, can find a way. Here's a look from a few years ago at how things are done these days:

https://www.youtube.com/watch?v=alADcidk7bs

The CEO in that clip has since been fired along with a few of his cronies, and his ambitious new-model plan is dead too, replaced by a more modest plan for an all-new Elise in a few years (the current Elise goes back twenty years) and maybe an SUV (THE HORROR!!!).

But here's the good ol' days. For all I know, my car might be one of the yellow Plus 2's we see going by in the moving shot of the row of parked cars.

https://www.youtube.com/watch?v=i0y7Gx98kQw