Master cyclinder leaks
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New User
Joined: Jul 2006
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From: Utah
Master cyclinder leaks
I have a 78' F350 4x4 Supercab, with a 400M. Since I've owned the truck I've had to fix a lot of little things to get it the way I want it. But the one thing that is bugging me are the brakes. I replaced the front right side caliper, both rear brake cylinders, brake booster, new brakes on the rear plus a spring kit. I had to replace some brake lines to the back, because they were pinched shut. I've had to replace the master cylinder 3 or 4 times, because it would leak out of the top. I noticed the other day it was leaking out of the top again. Does anybody know what would make them all leak out of the top. I'm thinking maybe its in the lines or brake proportioning valve? The brakes work pretty good, but I think they good work even better. I've bled the brakes several times and there isn't anymore air in them. It doesn't leak a lot of fluid, but enough I have to keep adding it to the front reservoir. You can see where it runs from the top down, but like I say it isn't a lot but enough. Any ideas, thoughts, suggestions would be helpful. Has anyone else had this problem?
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Joined: Oct 2008
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I have a 78' F350 4x4 Supercab, with a 400M. Since I've owned the truck I've had to fix a lot of little things to get it the way I want it. But the one thing that is bugging me are the brakes. I replaced the front right side caliper, both rear brake cylinders, brake booster, new brakes on the rear plus a spring kit. I had to replace some brake lines to the back, because they were pinched shut. I've had to replace the master cylinder 3 or 4 times, because it would leak out of the top. I noticed the other day it was leaking out of the top again. Does anybody know what would make them all leak out of the top. I'm thinking maybe its in the lines or brake proportioning valve? The brakes work pretty good, but I think they good work even better. I've bled the brakes several times and there isn't anymore air in them. It doesn't leak a lot of fluid, but enough I have to keep adding it to the front reservoir. You can see where it runs from the top down, but like I say it isn't a lot but enough. Any ideas, thoughts, suggestions would be helpful. Has anyone else had this problem?
Do have the gasket on the inside of the cap?
Post Fiend
Joined: Jul 2006
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What type of gasket are you using? Aftermarket gaskets tend to leak here, but original stuff seems to seal up well.
Something about the thickness, and the ability for the spring retainer to clamp the thinner gasket. Seems that it does not do a decent job, and the things leak.
Even old OEM gaskets can be used as long as they are not cracked, or brittle.
Something about the thickness, and the ability for the spring retainer to clamp the thinner gasket. Seems that it does not do a decent job, and the things leak.
Even old OEM gaskets can be used as long as they are not cracked, or brittle.
Posting Guru
Joined: Aug 2005
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From: Coal Country, ND
I've never had this problem. Maybe check is the "bail" (I think that's what it's called, the wire part that holds the cap on) to see if it's holding tight enough.
I've replaced 1 month old m/c's due to leakage at the rear where they bolt up to the booster.
I've replaced 1 month old m/c's due to leakage at the rear where they bolt up to the booster.
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From: Utah
The gasket is still in the cap. The clamp feels like its holding the cap good and tight, but maybe not tight enough. I think I'll try and replace the gasket and see what happens. It just seems weird that the caps coming with the master cylinders all have the same problem, thats why I wondered if it maybe it wasn't something in the truck that I was missing. 
Junior User
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Just to give you an idea, it's almost difficult for me to get the clamp to move from the cap of the master cylinder. That's how tight it should be.
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Fleet Mechanic
Joined: Feb 2007
Posts: 1,326
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From: Middle TN
I had one master cylinder that was leaking and when I examined it closely I thought that the top edge was too rough for the rubber to get a good seal. So I took it off and took a coarse file an filed the rim smooth. No more leaks.
To file it, I had a buddy hold it on the side of the work bench almost upside down so none of the metal fileings would fall into the resorvor and damage the internals, then I carefully washed it out to make double sure no metal particles were left inside.
To file it, I had a buddy hold it on the side of the work bench almost upside down so none of the metal fileings would fall into the resorvor and damage the internals, then I carefully washed it out to make double sure no metal particles were left inside.
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New User
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From: Utah
I've noticed the that top edge being rough too. I'm going to try that this weekend and see If I can get it a little smoother. The other thing I noticed was the the spring clamp wasn't as strong as I think it should be. On my other truck I had to use a flathead screwdriver and pry the spring back, With this master cylinder I can move it around with the palm of my hand without much force. I'll see if I can tweak the spring a little. Thanks for the great info!
Posting Guru
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From: Coal Country, ND
That's how mine is, it takes some real effort to get the clamp off.
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