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Discussion Starter #1
Hi everyone,
I have a 1967 GTO with all drums and no power boost.
I recently noticed a drop in brake fluid. I keep a catch pan under the car (just in case!) and thought it was coming from the distribution block. After a thorough search I could not find anything leaking. I had my wife apply pressure to the pedal and immediately noticed a tiny squirt out of a hole on the passenger side of the master cylinder. The hole is about a 1/4 inch diameter and all the way at the back of the cylinder, just in front of the mounting flange. Is this a breather hole and the fluid is showing a seal failure (fluid leaking past the seals on the plunger)? Or did something loosen and fall out of that hole such as a set screw plug? I can't get a good look in there to see if it has threads or not and I could not feel any with a screwdriver tip. I also can not find a picture of the passenger side of a master cylinder online. Can anyone explain that hole? From what I can tell, it might be the original master cylinder so a failure would not be unexpected.
Also, rebuild or replace? I'm thinking the price of a new one is relatively low so I might replace and after the summer, rebuild the original. Appreciate any help.

Thanks
 

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Shouldn't be weeping, so you have a failure.

"New" is probably rebuilt, or more properly "remanufactured" meaning the bore has been cleaned up before reassembly. You can rebuild yours or go the "new" route, all the same in the end (unless there are date codes or some such you want to keep with an "all original" motif)
 

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This master came off my 66' with the power boost. It does have a bleeder on the pass. side ... I installed a dual master with a set of discs on the front for safety reasons....Hope these pics help....J...
 

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Discussion Starter #4
JNYBOY,
Same location as your '66 but mine has no fitting in the hole as your picture shows. Maybe it fell out? The leak started suddenly and has not gotten worse. Hate to order a new one if it is just a fitting that loosened and fell out.
Good advice 64, will probably go the "new one " route unless it is just a fitting. Will check better tonight for threads in that hole.
Thanks all.
 

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Discussion Starter #5
This is the hole I am speaking of. The rear hole near the flange. It is on both sides of the unit. Almost looks like a locating hole for machining purposes so I'm really confused why fluid would come out there.
 

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This is the hole I am speaking of. The rear hole near the flange. It is on both sides of the unit. Almost looks like a locating hole for machining purposes so I'm really confused why fluid would come out there.
....That hole should not be a source of a leak, as it is a hole that goes all the way through to the other side. Not a bleeder hole, and no fitting or threads in there. ...Sure of the source of the leak ?....Best, J
 

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Discussion Starter #7
Gong to double check that tonight! Weird. When the pedal was pushed it actually blew out of that hole. More investigating to come.
 

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Discussion Starter #8
So after a better look with more lighting it seems as if the leak is originating around the reservoir top where the cap sits. The gasket/diaphragm on the cap looks good and the bar seems to snap it down tightly. I started by cleaning everything I could around the master cylinder, topped of the reservoir and took it for a quick spin. When I came back it was slowly dripping at that same hole but I noticed the passenger side of the reservoir was wet from the top down. I had not noticed that before. Can that gasket/diaphragm be the issue or am I somehow creating too much pressure within the reservoir? Can that gasket/diaphragm just be replaced?
 

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So after a better look with more lighting it seems as if the leak is originating around the reservoir top where the cap sits. The gasket/diaphragm on the cap looks good and the bar seems to snap it down tightly. I started by cleaning everything I could around the master cylinder, topped of the reservoir and took it for a quick spin. When I came back it was slowly dripping at that same hole but I noticed the passenger side of the reservoir was wet from the top down. I had not noticed that before. Can that gasket/diaphragm be the issue or am I somehow creating too much pressure within the reservoir? Can that gasket/diaphragm just be replaced?

....The G/D sounds likely. Replace the diaphragm (Cheap, no effort fix)...too much pressure ?, No Way !
Make sure that your retaining spring/clamp is a tight fit. ....Good Luck !
 

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Discussion Starter #11
Update:
I carefully cleaned everything and replaced the gasket/diaphragm with a brand new one. Took the car for a quick spin and when I got back the side of the MC was dry but there was fluid running down the front. Not badly but clearly wet. It was right where the tab for the gasket sticks out of the cover. I'm thinking the only thing left is the spring is not exerting enough down force to seal tight.
JNYBOY, you mentioned that in your last post. Is there a way to bend that to make it tighter or do I have to replace it?
It seems to clip into place tightly but I do seem to get a tiny bit of compression when I push down hard.

Thanks
 

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S,...for schlitz & grins, do a 180 with the cover, and put a little kink in the retaining clamp, which will put a smidge more downward pressure on the cap...If still not good, get another clamp as the metal might be a bit fatigued...Best, J
 

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Discussion Starter #14
Last night I did what JNYBOY suggested. Flipped the lid and bent the spring a bit and (fingers crossed) no leak after a twenty minute spin. The spring is much tighter and I am hoping that was the issue. I will keep the new MC on the shelf just in case though.
Thanks for all the help. This place is the best!
 
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