What causes this? (pic)

Hi guys. This is the second master cylinder cap that has done this. A buddy told me that it's bad brake fluid that causes this. Is the fix as simple as dumping and replacing the fluid?

Thanks

SimpleXJ

you using the right fluid? some of the other types (i.e. DOT 5, 4, etc.) could possibly cause this, perhaps someone saw that the reservoir was low (Quickie oil change place) when they did their advertised vehicle fluid level check, and topped it off with the wrong stuff,
Bleed it all out, bleed thoroughly from every wheel, and use manufactureres reccomended fluid or equivalent.

Unfortunately I am to blame, as my vehicles never see a shop. If the fluid is the culprit then I was the one who put it there

So what are we supposed to be running in these things? DOT3?

DOT3 and DOT4 are miscible, DOT5 is immiscible with either of the other two. Fortunately, DOT5 is fairly rare and there is/was a significant price difference, so you probably won't pick the stuff up by accident.

OTOH, brake fluid should be changed every other year! Best way to do that is with a pressure bleeder, second best (the way I'm doing it) is with a vacuum bleeder.

Draw all the fluid out of the master cylinder and clean the casting. Stuff probably looks like engine oil anyhow... Use brake cleaner to clean the crud from inside the master cylinder, and draw that off with the vacuum bleeder as well. Refill with fresh fluid from a sealed bottle.

In the standard pattern, vacuum bleed your brakes. Have a helper keep the fluid in the master cylinder up - if you suck up an air bubble, start over. Keep at the suction until the fluid comes out clear, then you can go to the next wheel. Bleed RR-LR-RF-LF, from the farthest wheel to the nearest (relative to MCyl.) Don't be afraid to clean out a lot - that's the idea here.

While you have everything opened up, HELP! (Motormite) and Dorman both make replacement cover gaskets for the Bendix master cylinders - and they're only a couple bucks. Yours probably wants changing anyhow.

You would know if you had mixed fuilds - I don't remember the effects, but they're pretty hairy.

A good choice for a replacement fluid is Castrol DOT3/4 LMA - it's somewhat less hygroscopic than most other DOT3/4 fluids, meaning it leaches less humidity out of the atmosphere. Water in the brake fluid makes for a spongy pedal and increases brake "fade" at high temperatures.

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Alright, so you guys agree that it's $hitty brake fluid. I have a brand new MC sitting here so I'll throw that on there and bleed the system. Hopefully I won't have to buy another cap seal after this.

Just out of curiosity, what exactly happened with the brake fluid causing the rubber seal to warp like that?


Thanks guys

SimpleXJ

this may sound weird- but:
the more water you have in your brake fluid, the easier it is to boil the juice.
your gasket got heated- thats what it looks like to me, anyway.

at 7% water, the boiling point drops down to 250 degrees, give or take(realitive theroy here)- superheating the fluid during stops- heat go`s up, an the gaskets where that heat stopped. remember, rotors get extremely hot under brakeing- water boils fast compared to dot3/4- and water converts to steam easily...steam dont stop squat.

actually, your probably saveing your own life (and others that may have to scatter out of your way)-- not to mention all those expensive parts that dont like water in them.....

thou shalt flush, bleed, and stop well. :wave:

What are your thoughts on using "Speed Bleeders"? And just using the brake pedal to pump out the old, while a buddy replenishes the MC with fresh?

speed bleeders are something i would buy if i was expecting to regularly need to bleed the system.. like frequent axle swaps and such... at $30 a set they are a bit pricey for me, but it is a convenience for sure..

ya- their neat. But they are for someone who does alot of bleeding, or needs a fancy quicky tool once in awhile

I use the age old tube in a bottle of juice. loosen the bleeder, put on the tube, put the tube in the bottle in the juice, pump till your done, tighten the bleeder. A one man job- but you do a lot of moveing back an forth, up an down.

works 100%, air cannot get back in, and its cheap. we like cheap. cheap is good.

An Ive seen speed bleeders leak.

Get the vacuum kit and use it - it does a better job, you can keep things moving consistently, and it works faster than the old "pump and jump." For the money, you'll think it paid for itself by the time you finish the job...

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