Master cylinder lid seal 68 dart

[1968 Dart 270]

Hi guys,my 68 dart is leaking brake fluid from the master cylinder cover. I pulled it off and the seal is still there,I cleaned it off,reinstalled it making sure it was positioned correctly and it still leaks. Since there’s nothing wrong with the master cylinder I’d hate to replace it just for the seal but so far none of the parts places I could find carry the seal.
Here’s a photo of the master I have. You can see the fluid leaking under it

 

[Inertia]

I have often found the cause to be a violent return of fluid to the reservoir caused by poor brake shoe adjustment with the springs returning
the shoes an excessive distance, but most often air in the brakes that compresses when you apply the brakes, then violently shoots fluid BACK in a fountain/geyser when the pedal is released.
The cap was designed to control sloshing around, not a forced jet of fluid.
To check, have someone pump the brake pedal a coupla times and hold it down hard.
Remove the m/c cap, watch the fluid , and have the pedal released.
There should be slight fluid return to both systems .
I suspect one will have a heavy return that, if spashed on the paint, will ruin the paint, rinse with water IMMEDIATELY ! !
Adjust and/or bleed the system with the heavy return .
Good luck .
Cheers

 

[Oldmanmopar]

Rebend the clamp that holds the lid on to give it more tension. Just put it upside down on a hard surface and hit the middle down.

 

[Gr8polarbear]

Had the save problem with mine. What i ended up doing was getting a pack of make your own gasket and cut out a double gasket and put in between the lid and mc. Something like this
https://www.autozone.com/sealants-g...VEv7jBx10uQSrEAQYASABEgJZxPD_BwE&gclsrc=aw.ds

 

[BillGrissom]

Common, and why every A-body I've seen in the junkyard (or bought) had a rusty firewall under the MC (great paint remover). The cap needs to be airtight to keep moisture from the air being absorbed in the glycol fluid (DOT 3, 4, 5.1) to cause internal corrosion. Less concern w/ silicone (DOT 5), but wouldn't want that expensive fluid sloshing out.

Even with a good cap, you should flush thru new fluid say every 4 years in eastern U.S. (maybe 10 yrs in west). Newer MC's have a flexible rubber diaphragm to allow volume changes without sucking in outside air. Most old caps just have a circumferential gasket, so perhaps need to exchange outside air as the fluid level changes (to not draw a vacuum). Many people toss the rusty cast-iron MC for a newish alum one like for a 1980's Dodge truck. Those are 2-bolt so need a 2 to 4 bolt adapter plate. Can buy on ebay, or Dr Diff sells an MC w/ plate.