leaking master cylinder cap.

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doopdoop67

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Whats up guys? I have a 67 dart that I put 72 disc brakes on, every master cylinder i've had on it leaks around the cap. I tried a bunch of stuff to make them stop leaking but no luck. it drips down and leaks in my TTI headers. any advice? i was thinking maybe a plastic reservoir with the screw on caps but not sure what would fit. I run the brakes manual
 
I have the same problem and did replace mine with the aluminum one with plastic caps from Dr.Diff and it still leaked although not as bad as the iron unit.

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That’s odd. I’m using the Mopar performance light weight cylinder with no issues, other than the cap will leak if I have it jacked up too high in the front or rear.
 
That’s odd. I’m using the Mopar performance light weight cylinder with no issues, other than the cap will leak if I have it jacked up too high in the front or rear.
Yep that’s understandable, I have mine half full trying to make it quit dripping out the cap. Still leaks! I think what happens is when you hit the brakes hard it sprays straight up and leaks out of the seal. I’m about ready to just make a drip pan for it. Not sure what else to do
 
I had the same problem when I did my brakes. I ended up using the stock gasket and made my own gasket more like a ring that just went around the flange on the inside of the cover. I think think the fluid was too high and removing just a little bit fixed it. I thought the metal clamp wasn't tightening it enough.
 
Maybe you have done this already, but I draw file the top of the m/c, try to get a new gasket (older gaskets tend to get harder) and increase the pressure from the bale.
 
Maybe you have done this already, but I draw file the top of the m/c, try to get a new gasket (older gaskets tend to get harder) and increase the pressure from the bale.
I actually tried that before. It helped some but still leaks. Thank you though
 
I had the same problem when I did my brakes. I ended up using the stock gasket and made my own gasket more like a ring that just went around the flange on the inside of the cover. I think think the fluid was too high and removing just a little bit fixed it. I thought the metal clamp wasn't tightening it enough.
Sounds like a good idea. Maybe a little extra gasket would help. I bent the clamp a little to give it a little more tension but still leaks! Driving me crazy ! lol
 
Oh man, maybe I’ll steal your idea and install some type of material to absorb the fluid and I’ll replace it as needed.
That's ok because It's not my idea, something I saw on the internet that was meant to be a heat shield.
 
First thing I would do is get a new gasket. Second I would be sure the level of fluid is low enough that when the car is on an incline, (perhaps the most incline you encounter) the fluid is below the lip of the cylinders.

Lastly clean and dry the lip of all fluid.
Clean the gasket of all fluid and debris.

Put the cap on as centered as possible and snap the retainer on. Let it sit that way over night.

Now drive around, hard stops, inclines etc. And check for leaks
 
Maybe you have done this already, but I draw file the top of the m/c, try to get a new gasket (older gaskets tend to get harder) and increase the pressure from the bale.

There was another thread I replied to about a week ago that claimed same gasket malady. I suggested a perfectly smooth and level housing surface can be achieved by removing unit and dressing it using med - fine emery, or similar. Invert unit on flat (glass, plexi) surface, similar as when restoring aluminum t-stat housings. Rinse out insides when done - ie: inverted and brake-clean. Black gasket/seal should be soft enough to dig a fingernail into. Extra step would be to use some type of sticky sealer, just a teasing hint, on metal housing perimeter. It only takes a bit and no chance of contaminating the fluid. I have also used vaseline with no concerns. If you attempt to dress contact surface while unit is installed you risk having residue enter the system. In these situations, draw out most of the fluid and stuff some paper towel into the chambers. Then, vacuum out when done. I've had many new or reman units that had rough surfaces - right out of the box. :)
 
I have the same problem and did replace mine with the aluminum one with plastic caps from Dr.Diff and it still leaked although not as bad as the iron unit.

View attachment 1715712747

The caps and gaskets can be replaced. I've found that they will develop small leaks over time, or after you remove/tighten them a bunch. The other thing is that the bump on the cap corresponds to a small vent, so, point those toward the rear and not the front. The caps (#42044) and gaskets (#42081) are readily available, and fresh ones should stop any small leaks you may have.

IMG_0382.jpg


Dorman 42044 Dorman Master Cylinder Caps | Summit Racing
Dorman 42081 Dorman Master Cylinder Reservoir Cover Gaskets | Summit Racing
 
Mine weeps from the caps sometimes. Master was bought new. Don't mind the wiring mess. Had to do emergency hacking when MSD **** the bed .Gettin ready to rewire and keep both. Stock for backup.

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Could your system be blocked causing the pressure to come back through your master cylinder. My distribution block was plugged causing it to leak out the lid same as yours.
 
Lots of good ideas already mentioned like flattening the mating surface of the MC with a file.

As a last resort, maybe you could get a second gasket and cut it apart leaving a half inch or so around the circumference. Use that to double up the clamping area.

You might try contacting the Ramman. Maybe he could shed some light on this.

I run DOT5 in all my old cars. One of the advantages is that leaks or spills don't eat the paint, just wipe it off.
 
I've found that for the cast iron OEM masters, the gaskets that are available are thinner and of a more 'plastic'-y material. When I was still running the OEM master on my '70 Dart I found this one from IH is just like the factory 'originals' in thickness and composition: Disc/Drum Brake Master Cylinder Cap Gasket for 1974-80 Scout II, Terra or Traveler - IH Parts America . It solved the leak problem.
Often the same casting masters were used across a variety of vehicles and you can search for a gasket by the casting number.
 
I've had dozens of cast-iron or aluminum/plastic cyls. over past several decades and none ever leaked. Only issue has been one or two wearing out inside and rear seal weeps down firewall. With the pre- '80-ish cast iron units. the large black seal must fit perfectly around reservoir perimeter. It can only leak fluid at that surface. The lid is only there to clamp down hard onto said seal and does not have to fit perfectly on surface where seal contacts lid. The design of the seal allows it to 'flex' downward as fluid levels decline over time, so to prevent air from entering the reservoir. When removing the lid and fluid is way down, the seal should be pulled downward - as it is designed to. Therefore, cover itself has to be vented. Adding a second open type gasket around the reservoir perimeter (under main seal) should not be necessary. (I'm trying to be polite here). I suggested permanent fixes in my previous post. I also compared to the OEM aluminum T-stat housings. Which would be more prone to coolant leaking? - a dry gasket or one with sealer on it? The (Red-Green) idea of putting a metal tray underneath? - (sigh). IF anything, at the very most, just cram a large, soft rag underneath reservoir - - until you can fix the problem, or, return unit for a refund or replacement. :thankyou:
 
The caps and gaskets can be replaced. I've found that they will develop small leaks over time, or after you remove/tighten them a bunch. The other thing is that the bump on the cap corresponds to a small vent, so, point those toward the rear and not the front. The caps (#42044) and gaskets (#42081) are readily available, and fresh ones should stop any small leaks you may have.

Just wondering, what is the reason for pointing the vent toward the rear?
 
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