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Brakes suddenly act up and go soft...

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dkgraber85 Avatar
dkgraber85 Darin Graber
SLC UT, USA   USA
1955 MG TF
1957 Austin-Healey 100-Six "Brutus"
To the forum,

My '57 100-6 BN4 has been running great all summer - over 5000 miles right now. Today, while driving, the brakes suddenly went soft. So much so that I had to pump three times to get pressure to stop while limping back home. When I got home, I checked the master cylinder thinking I would find the level low or gone. Just the opposite...the reservoir was over filled to the point that fluid was squeezing out the cap and onto the exhaust. At a loss. Any ideas where to start? Thanks.

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mudbob Avatar
mudbob Bob Greenslade
Roseville, CA, USA   USA
Sounds like the master cylinder is bypassing internally and needs to be replaced or rebuilt.

dkgraber85 Avatar
dkgraber85 Darin Graber
SLC UT, USA   USA
1955 MG TF
1957 Austin-Healey 100-Six "Brutus"
So tonight took out the mc to see what had happened. It looks perfect. Rubbers are all good, etc. Honed the bore, lubed and reassembled. Then bled the whole car and brakes are back. Not sure what happened. Still a bit soft, but the mc works as it should, and I think things will settle in again. Maybe some dirt?

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San Jose, California, USA   USA
Did you inspect the 'foot valve' closely? This is the small, spring-loaded seal at the front of the MC piston mechanism which allows the reservoir to fill the MC when the pedal is released, but seals immediately when the pedal is depressed. I had one degrade (torn?) somehow on a long road trip, and had to pump the brakes 2-3 times to get pedal every time I anticipated braking. Sometimes, you can detect this condition by watching the reservoir level while someone applies the brakes; if the seal is leaking you might see the level rise a bit, as you're just pushing the fluid in the MC back into the reservoir. I think it's possible there was some crud in the reservoir or MC that prevented the seal from sealing properly; you might have gotten it out when you rebuilt the MC, but there might still be some crud in the reservoir and/or the reservoir->MC line.



Edited 1 time(s). Last edit at 2017-10-07 11:24 AM by Bob Spidell.

Michael S Avatar
Michael S Michael Salter
Dwight, Ontario, Canada   CAN
Hi Darin,
You mentioned Honed the bore, lubed and reassembled. not good.

Most people do not realize that the internal bores of aluminuim brake cylinders are anodized to provide a hard wearing surface. Honing the bore of these cylinders removes the hardened surface and promotes rapid wear. You will know this is happening when your brake fluid starts to turn black as a result of the material wearing off the bore and the rear shoulder of the cylinder seals being shredded away because they are extruding into the gap between the enlarged cylinder bore and the the piston.
I think Bob's analysis is very likely correct although the failure of a brake adjuster, most likely a front one, can produce identical results.
Consider replacing the master cylinder ... brakes are not something to be complacent about because they typically fail when you need them most!!



Michael Salter
Technical Chairman (Big Healeys)
Austin Healey Concours Committee

wagonaire144 Avatar
wagonaire144 bill d
mequon, wisconsin, USA   USA
Darin,
you didn't mention ( that i could find...) the type of fluid you are using. is your car set up standard, or have you fitted disc front brakes?
In my experience, when there is too much fluid in the reservoir and i didn't put it there, it is a case of overheated brakes, which makes the fluid expand, and further applies the brakes, which leads to boiling fluid, and a mushy pedal. Once cooled off, the level goes down, the brakes operate like normal. If the fluid was older, at the start of your trips, it could contain enuf water from condensate/ weather changes to drop the boiling point.
Also, if using silicone fluid, the brakes could get hot, and the pedal feel doesn't change... it just gets hard, then goes to mush.
What makes them get hot? where you using them in a spirited way, or just noodling? If all 4 are drums, how old are the return springs?
Good luck in your search, and let us know what you find!
billd

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