Mystery brake leak at new MC

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Bobacuda

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I installed a new Raybestos MC with new brake lines - and a persistent leak on our 67 Barracuda. Now, if I let the car sit several days brake fluid is on the bottom of the MC and dripping off - which messed up the new paint under the hood until I wrapped a rag around it.

My problem is that I cannot locate the source of the leak. It does not appear to be getting past the MC cap, and I think I finally have the lines not leaking.

However, I have noticed the “bolt/plug” on the bottom-middle of the MC. I have looked and cannot find a MC cross-section to refer to, so I have no idea what that bolt/threaded plug is, or what it’s function is. Can I tighten it? Can I remove it and see if it has a missing or failing washer?

Any ideas what it is?
Thanks,
Bob
 
Been a long time I've had one of those apart. Take a look at the factory service manual. You can download them free at MyMopar. Some of them there got there from the guys right here. You may have to settle for Dodge vs Plymouth or vice versa for some years
 
I had a similar mystery leak on a new master cyl, kept thinking it was my brake lines / flares that I made, turned out to be a flaw / crack in the casting! Only found out because I shoved my camera under there and took a pic before I wiped it off one day!

F8DBDBBA-33BB-4249-B698-8D967A541797.jpeg


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If i recall, the bolt is a retainer,or a stop for the piston.
I would dry it and see if leak is bolt or coming from push rod end. Or give bolt a little snugging. Never know, could be loose. But dont pull bolt out unless you have it on the bench.
Be aware, parts could pop out.
 
I have the same problem on a '65 Valiant convertible; that drain plug thing at the bottom of my new single master cylinder is always wet with a drop or two of brake fluid. I kept tightening it over and over again and the minute "drip" never stopped.

So I purchased a second new master cylinder. But this time I first removed that plug, used some 3M thread sealant on it, and tightened it back up. Now I can't believe I have the same problem again. I have tightened the "plug" one or two times since and still the same damp/wet spot in the same place under the little plug.

I don't see it overflowing from the cap or dripping "down" from the single brake line either. Driving me crazy. I don't know if these two "Autozone" master cylinders I purchased are using original Chrysler castings or if they are reproductions.

Oh, by the way: the reason I replaced the master cylinder (and all the brake cylinders) is because I found the master brake cylinder was empty/dry after having not driven the vehicle for a very long time. I don't know for sure how or where the fluid leaked out to begin with (the master cylinder that was on the vehicle and found empty I had replaced about ten years ago).
 
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Those stop plugs should have a copper washer under the head. The copper being soft, will compress & form a seal.
 
Bewy said:
Those stop plugs should have a copper washer under the head. The copper being soft, will compress & form a seal.
Click to expand...

I have to disagree, a washer for a "plug" that only goes in so far has no application, at least in this case (same thing goes for the coolant drain plugs in the cylinder blocks).

I see the '65 Factory Service Manual does not even picture the plug in the parts blow-up diagram.

Screen Shot 2022-09-03 at 3.48.34 PM.png
 
dibbons said:
I have to disagree, a washer for a "plug" that only goes in so far has no application, at least in this case (same thing goes for the coolant drain plugs in the cylinder blocks).

I see the '65 Factory Service Manual does not even picture the plug in the parts blow-up diagram.

View attachment 1715980251
Click to expand...

The "plug" is a stop for the front half of the piston.
All the ones I've seen, dozens, have a copper washer on them, and was regularly included in the overhaul kits.
Caution when installing "plug/stop" is if the front piston is not properly in place, damage to that piston and seal may occur .
Block plugs are tapered pipe thread .
 
To my knowledge, there is no reason for a plug to be included in a rebuild kit and I have never seen one. I think we are discussing two different areas of the master cylinder.

Screen Shot 2022-09-03 at 3.56.42 PM.png
 
Here are some fotos of the master cylinder "plug" I have been discussing that I just removed from a master cylinder that was installed on a '65 Valiant for the past ten years. I see no function for the plug except to fill a threaded hole and no washer (the plug still shows 3-4 threads when fully seated/tightened).

IMG_0875.jpg


IMG_0876.jpg
 
Have you checked in the passenger compartment, where the push rod goes thru firewall? Ours had a leak that was related to the pushrod area...did same thing to paint...immediately switched to Dot 5.
Bobacuda said:
I installed a new Raybestos MC with new brake lines - and a persistent leak on our 67 Barracuda. Now, if I let the car sit several days brake fluid is on the bottom of the MC and dripping off - which messed up the new paint under the hood until I wrapped a rag around it.

My problem is that I cannot locate the source of the leak. It does not appear to be getting past the MC cap, and I think I finally have the lines not leaking.

However, I have noticed the “bolt/plug” on the bottom-middle of the MC. I have looked and cannot find a MC cross-section to refer to, so I have no idea what that bolt/threaded plug is, or what it’s function is. Can I tighten it? Can I remove it and see if it has a missing or failing washer?

Any ideas what it is?
Thanks,
Bob
Click to expand...
 
Back to the mystery brake leak.

1967 Barracuda Formula S.
Power brakes, front disc.
New cast iron dual chamber MC, the bolt on the bottom with the copper washer. Does not leak at the rod.

Last week I zip-tied a rag to the bottom and by the lines out of the MC to help isolate the leak. Needless to say, it hasn’t leaked since then. No idea why, but I’ll take it.

I guess it does not like a wet diaper.
 
dibbons said:
To my knowledge, there is no reason for a plug to be included in a rebuild kit and I have never seen one. I think we are discussing two different areas of the master cylinder.
Click to expand...

I do beg your pardon dibbons .
For whatever reason I was thinking dual reservoir M/C .
My apologies.
 
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Referring to the original post. It is a dual m/c & the poster specifically refers to a bolt, bottom-middle. That is the piston stop for the rear piston & should have a copper washer for sealing because it is a parallel thread, not a tapered thread. The bolt may not be included in o'haul kits because it is a non wearing item. Bolt is not needed on single m/c.
 
Aware of that, but if you read the original post [ & that is what I have been replying to in this thread ], the OP had a leak under the m/c & could not see a washer under the screw/bolt head.
 
Thanks Bewy,
I got back from the track, where we won the main, and clinched the championship, clicked on dibbon's "Alert", before reading the rest of the thread, - not realizing the subject M/C had changed.
I was referring to the copper washer being supplied in the O/H Kits, not the screw .
Another race today.
Hope you folks sort this mess out .
Cheers .
 
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Cal Tonsley said:
I had a similar mystery leak on a new master cyl, kept thinking it was my brake lines / flares that I made, turned out to be a flaw / crack in the casting! Only found out because I shoved my camera under there and took a pic before I wiped it off one day!

View attachment 1715977364

View attachment 1715977365

View attachment 1715977366
Click to expand...

If you're dumb enough to install it on the wrong side of the car, who knows what you did to it during the install.....:realcrazy::realcrazy:


What the hell are you driving, anyway?
 
jos51700 said:
If you're dumb enough to install it on the wrong side of the car, who knows what you did to it during the install.....:realcrazy::realcrazy:


What the hell are you driving, anyway?
Click to expand...
Ha, Ha….it’s a 1970 VF Valiant Hardtop (built by Chrysler Australia as a local RHD version of the 67-69 Dart). Originally sold with a 318/904 now with 410/727. It’s been a long project, hope to have it running soon.

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Try fitting a big block [ I did it ] & it gives new meaning to the word 'nightmare'...
 
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