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MG Midget and Sprite Technical - Rear brake leak... again!

I was backing into my garage the other day when I heard the right rear brake binding. Nuts... so when I got a chance shortly after, I pulled the drum for a look and found it full of brake fluid. Nothing new there, since I've dealt with leaky brake cylinders - and swapped them for new - in the past. But this one was way leaky, to the point where it was leaving a puddle under the tire when parked.

So I ordered new linings and cylinders for both sides, and took a day off today to work on them. Turns out the RR cylinder was pretty rough inside, obviously due to the usual moisture condensed from air bubbles. I hadn't bled the brakes for a while, but the last time I did I gave them a good bleeding, so I figured I wouldn't have to worry about them for a while. Wrong...

So what's with that? Due to the harsh weather this past winter, the car sat most of the time... is it this lack of use that causes bubbles and condensation? Will driving more often reduce the frequency of bleeding?

By the way, I ordered my new brake parts from NAPA this time, just for a change. The cylinders appear to be of good quality. Funny thing is, they list two sets of brake linings for the midget rear. I picked the one that was a little less expensive, only to find this morning that they were too large. A look back at the NAPA website revealed that the two sets they list are actually two different sizes... one for 7" drums, and the other for 8". Did the Midget ever use 8" drums? I thought they were all the same size. I rather suspect I may have gotten a set of MGB linings, but they're sure listed under Midget.

So now the car gets to spend the next few days up on jackstands waiting for new linings. Got the cylinders done.

-:G:-
Gryf Ketcherside

Gryf,
What frequently happens is that when the car is parked up for a while rust starts to form on the swept portion of the cylinder outside of the seals. In other words, the entire area of travel for the seals is basically unfinished metal waiting for a chance to rust. Regular use keeps the rust wiped away before it gets any body to it. Lack of use allows the rust to get enough of a toe-hold that resumption of use promptly wears out the seals and you have a leak. Miata owners who park their cars for the winter find this out with the clutch slave cylinder.
David "Never park your Spridget too long" Lieb
David Lieb

indeed cars just sitting in the gagare hav all sorts of strange failures

so buy a hardtop and keep driving!
Onno Könemann

David -

Interesting observation! Yes, when I looked in the cylinder bore, I could see that it wasn't really rusted much in the middle, between the pistons - the damage is mainly outboard of the seals. And this would also explain why that brake was stuck and dragging when I got home the other day... I'd applied it (to stop, as I recall...), but due to the roughness, the seals were grabbing and not letting the linings retract all the way.

Oddly, the other brake was as dry as a bone, and the cylinder is nice and smooth inside. Go figure. Well, now that I think of it, the one that failed had a split dust boot. Not sure how THAT happened, but perhaps it let sufficient moisture in that it caused rust in the bore.

Thanks again - from here on out, I'll make sure to work the brakes more often, preferably by driving. Just not on ice or salty slush, as was too often the case last winter!

Cheers,

-:G:-
Gryf Ketcherside

This thread was discussed on 26/06/2009

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