Brake Master Cylinder Lid Leaking

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Lord Sparky

Lord Sparky

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Have a 2-year-old Leed Brakes LEB-A8471 master cylinder that started leaking from under the metal lid. The gasket looks good and the lid does not look deformed. Led sent me a new rubber gasket but it still leaks. The brake fluid in the larger bowl is 3/8” below the rim which Leed says is correct. The fluid gets up over the top of the gasket (see no holes) and drips down the sides. I thought about making a thin plastic shim to put under the metal cover to exert more pressure on the gasket. Any other ideas? Is the fluid too high?

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Take a metal staight edge ruler and lay it length ways across the mating surface of the Master cylinder. This should indicate if the surface is level or slightly higher or lower resulting in a loss of mating surface and therefore subsequent leaks.

From the pic it looks like the leaking is possibly occurring to the right of the stick you are showing in the larger bowel. See the brown residue??
 
Maybe run a fine file over it to remove any burrs or high spots. I bended my hold down wire on mine that sometimes weeped for added clamping pressure. Worked for me.
 
Common concern for these.

So far no one has come up with a solid reason or solution.
 
It is a rebuilt master that could have an internal pressure leak due to a bad front piston seal. This will cause the front cavity/rear brakes which apply first to pressurize the rear cavity/front brakes. Press hard on the brake with the lid off while someone is watching with a rag in hand. I have seen this in the past . Very rare but it does happen.

Also the new rubber gaskets on this reproduction masters are not rubber. They a different material and should not be used with Dot. 3

The seals are a different material. They are very shiny and slippery. That also could be the problem. I see yours is square in the front. That is definitely a reproduction not a rebuild .

Read the info below page 3 caption. The newer reproduction master cylinders use a different fluid or they do leak

http://www.moparts.org/Tech/MoparAction/Master_Cylinder.pdf
 
Oldmanmopar said:
It is a rebuilt master that could have an internal pressure leak due to a bad front piston seal. This will cause the front cavity/rear brakes which apply first to pressurize the rear cavity/front brakes. Press hard on the brake with the lid off while someone is watching with a rag in hand. I have seen this in the past . Very rare but it does happen.

Also the new rubber gaskets on this reproduction masters are not rubber. They a different material and should not be used with Dot. 3

The seals are a different material. They are very shiny and slippery. That also could be the problem. I see yours is square in the front. That is definitely a reproduction not a rebuild .

Read the info below page 3 caption. The newer reproduction master cylinders use a different fluid or they do leak

http://www.moparts.org/Tech/MoparAction/Master_Cylinder.pdf
Click to expand...
Great article! I will try the test you suggested. Leed said to use DOT 3 or 4. The master cylinder lid is stamped “Use DOT 3”.
 
I had this same problem. The lid wasn’t centred on the master cylinder. Once I centred the lid the leaking stopped.
 
Mine always leaked. I wondered about putting a small bead of silicone around the edge of the lid and letting it set up. Then replace the rubber gasket. I figured it would put more pressure on the gasket, but I sold the car before I tried it. For my current car I bought a late model plastic tank master, just because I was sick of the leaks.
 
Mine was leaking and I bought another master cylinder that seemed to have a smoother surface on the cast iron portion and leak went away. My previous master cylinder the cast iron surface wasn't smooth seemed rough porous.
 
I bought some make a gasket materiel and cut out two new gaskets the same as what was on it and doubled it up and that stopped the leak
 
First off, beautiful engine bay. Mine leaked at the lid, I found there was a clog in my brake lines.
 
I have the same issue. I ran a bead of silicone on mine. It stopped the leak but when you want to check it you have to reapply, not good if you’re on the road. It ruined my paint job and headers from leaking. I’m in the market for a new one.
 
On several occasions like yours, I found the brake system to have air in it, and when the brake pedal is released, the compressed air forces fluid back to the reservoir, and there is a violent fountain of fluid shoots up against the cap, and some migrates under the cap. I know some that should know better will disagree, but that has been my experience, and bleeding the air out of the system cured the issue, not a new seal, top, or M/C.


Want to check it, have a friend pump the brakes a few times and hold the pedal down, THEN remove the cap and have the pedal released, if you get a prolonged fountain of fluid squirting back into the M/C, then you need to stop the fluid return, by adjusting brakes, and bleeding.. jmo


PS, have plenty of water nearby, to immediately rinse any spilled fluid off the paint, as brake fluid is excellent paint remover.


hope it helps
 
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