Brake boil over

[Evan Dutch]

Hey guys,

Has anyone experienced losing brake fluid through your master cylinder reservoir caps? Just installed a new manual brake master cylinder and a front and rear disc brake kit from Dr diff. I notice that my brake fluid is overflowing, I’m not sure if it’s because it’s boiling over or if it’s because the cap seals are bad? When the car is on a slight incline/decline/hill/etc… Brake fluid seems to leak from the caps. The reservoir is not overfilled.

My second thought is that the exhaust manifold could be overheating the master cylinder? It looks awfully close.

Anyone ever have a similar issue?

 

[72bluNblu]

I doubt there's any "boil over" involved unless you've experienced a ton of brake fade when driving. The gaskets that come on those master cylinders are kinda lousy. If you open and close them a bunch (like during the bleeding process) it seems like they start to leak.

You can buy replacement caps and gaskets for them, I did that with mine and haven't had any issues since. Dorman #42044 for caps, #42081 for gaskets

Also, on the cap that raised section of plastic on the cap is a vent, and it works best if that's at the highest point- ie, don't put that facing the front of the car, because when you stand on the brakes the fluid shift will put fluid out the vent.

Like in this particular picture, your vents are facing mostly forward...

 

[Evan Dutch]

I doubt there's any "boil over" involved unless you've experienced a ton of brake fade when driving. The gaskets that come on those master cylinders are kinda lousy. If you open and close them a bunch (like during the bleeding process) it seems like they start to leak.

You can buy replacement caps and gaskets for them, I did that with mine and haven't had any issues since. Dorman #42044 for caps, #42081 for gaskets

View attachment 1716209912

Also, on the cap that raised section of plastic on the cap is a vent, and it works best if that's at the highest point- ie, don't put that facing the front of the car, because when you stand on the brakes the fluid shift will put fluid out the vent.

View attachment 1716209913

Like in this particular picture, your vents are facing mostly forward...

View attachment 1716209914

Thank you very much. I had no idea about the vents. I’ll go ahead and order those gaskets (the caps don’t really have much resistance to them when putting them on). And maybe order a heat shield

 

[72bluNblu]

Thank you very much. I had no idea about the vents. I’ll go ahead and order those gaskets (the caps don’t really have much resistance to them when putting them on). And maybe order a heat shield

I wouldn’t worry about the heat shield. It’s probably just a combination of lousy gaskets and the vents on the caps.

I’ve got headers on my car with the same master cylinder and don’t run a heat shield, the gaskets and pointing the vents toward the rear of the car solved the leak I was having.

 

[Evan Dutch]

I wouldn’t worry about the heat shield. It’s probably just a combination of lousy gaskets and the vents on the caps.

I’ve got headers on my car with the same master cylinder and don’t run a heat shield, the gaskets and pointing the vents toward the rear of the car solved the leak I was having.

Thank you very much

 

[MoparMike1974]

You might be over full in the reservoir too. It shouldnt be jammed packed to the top.

 

[mrhp]

I doubt there's any "boil over" involved unless you've experienced a ton of brake fade when driving. The gaskets that come on those master cylinders are kinda lousy. If you open and close them a bunch (like during the bleeding process) it seems like they start to leak.

You can buy replacement caps and gaskets for them, I did that with mine and haven't had any issues since. Dorman #42044 for caps, #42081 for gaskets

Replacing the gaskets makes sense. But the caps look identical to the ones that are already on the master. Wondering why the need to replace the caps too?

 

[72bluNblu]

Replacing the gaskets makes sense. But the caps look identical to the ones that are already on the master. Wondering why the need to replace the caps too?

Because they’re just cheap plastic and the tabs seem to flex, which keeps them from putting enough pressure on the gaskets.

You can try just changing the seals and see if that works, but if it doesn’t I’d change the caps next.

I’m not sure what it is, the caps should be a non-wearing part but it seems like if you take them on and off a bunch (like if you’re changing a bunch of brake parts and bleeding them a few times) they just don’t put enough pressure on the gaskets anymore.

 

[mrhp]

Wondering… It appears that Mopar discontinued the black caps in favor of the yellow ones. Are the yellow ones made of a stronger plastic, less prone to flex?

 

[Inertia]

If there is still air in one of the systems, when you press the pedal, it compresses the air, when you release the brake, the compressed air can shoot a violent return to the master, blowing fluid past the caps/vents.
Set the emergency so the shoe springs can't return the shoes.
Have a friend pump the brakes a coupla times and hold the pedal down, remove the M/c caps, and have the pedal released.
If you have a fountain of fluid return, - that's likely air shooting fluid back.
That system needs more bleeding.
Rinse any brake fluid off the paint immediately, as fluid is excellent paint remover!

 

[mrhp]

Because they’re just cheap plastic and the tabs seem to flex, which keeps them from putting enough pressure on the gaskets…

Just to clarify, my DrDiff master has been on a couple years now. Noticed very small seepage from the forward cap after driving Friday. Just picked up the 42044 and 42081. Surprisingly to me, stronger material caps, thicker gaskets, snugger fit. I’m sold.

 

[goldduster318]

The newer caps with the vents like this - you need some air space for expansion to happen. You don't need the fluid all the way up to the bottom of the openings for the caps. You want at least 10% of the total reservoir volume for air space.

But yes, the vents toward the back is very important.

Wondering… It appears that Mopar discontinued the black caps in favor of the yellow ones. Are the yellow ones made of a stronger plastic, less prone to flex?

It makes no difference, it's just a color. They like to make safety critical items bright colors now. Most cars after a certain time in the 90s have to have transparent brake fluid reservoirs (plastic) with the max fill line clearly marked.