Leaking master cylinder mystery

[AR67GTX]

This is a weird one for me. Overhauled the brakes on my 66 this spring and bought a reproduction master cylinder since the bowl in the old one was pretty rusty. The new one had a tapped port in the boss on the bottom with a plug in it that the old one didn’t have. The boss was there but no port. Tightened the plug securely, bled it, put it on the car, bled the brakes, etc. After every drive I would find fluid collecting on the bottom of the boss and no sign of it on the line or upper MC body.

So I pulled the plug, applied teflon tape, reinstalled and still a leak.

Pulled the plug again, pulled the plug, removed the tape, put Gas Cinch thread sealer, tightened up and still a leak.

Pulled the plug, cleaned out the Gas Cinch, applied Permatex #2 and tightened up and still a leak.

Pulled the plug again, put a small o-ring in the bottom, reinstalled the plug and still a leak, always on the bottom of the boss.

Bought another reproduction MC from different source and Hallelujah, the boss is not tapped, and is just like the old one. I like the finish of the cap on the first one better so I swapped it to the new one. Bled the MC, installed, bled brakes, went for drive. Dry when I return. Go to the garage the next day and bottom surface of boss is wet. No sign of any wetness on the line, the line adaptor, the cap or the upper MC body. WTH!

OK, I can’t see any sign of fluid on the upper body but the cap must be seeping slowly. So, I cut a 3/16” circle of cork gasket to glue to the bottom of the cardboard gasket provided for the cap and I put a sealing washer on the center bolt. Tighten it down. Go for a drive, dry when I return and later in the afternoon, next morning the bottom is wet again. Sometimes it’s even on the second day after a drive I find fluid.

Tighten the cap bolt tighter, drive, wet the next day on the bottom of the boss.

Tighten the cap tighter, drive, wet the next day on the bottom of the boss.

Repeat tightening but by now I realize I’m collapsing the cap.

Pull the silver cad cap out that came with this MC and installed it. Pushed the top out on the first repro MC for posterity sake. Haven’t driven it yet - so damn hot to go for a drive in an all black car.

Has anyone run into this??? There is absolutely no sign of leakage that I can find. The fluid just mysteriously appears after a day, sometimes 2 days on the bottom of the boss. The line is tight but even if it were a line I believe I would see fluid immediately after a drive from high line pressures, not a day or two later. No sign of leak from the rear from either MC and it would have to run uphill to get on the boss. I thought about a casting leak but what are the chances of 2 MCs with the same casting flaw? Besides after a couple days I can clean the bottom off and it will then be dry until I drive it again.

I think I’m going to see if I can find an o-ring that fits the inner perimeter of the cap to seal on the MC bowl lip. But I’ll have to be careful that it doesn’t squeeze out of position.

Any ideas what’s going on and how to stop?

 

[JonR]

I had the same thing with my 66 pot. I always thought it was from a fitting or that bottom boss. Finally it had to be that the cap was weeping slowly down the sides. I sprayed the pot with brake clean to get it perfectly dry and eventually saw the very slow weeping. I cut a new gasket for the cap and added a rubber washer to the hold down bolt. Then I tightened the bolt more than I wanted to because of possible cap distortion. That got most of it but I just can't stop it all. It's much slower now and I just brake clean the master every few weeks.

 

[Fran Blacker]

If you didn't have problem with your original have it sleeved. Is the surface where the cap goes on machined or as cast? Two pours castings don't think so.

 

[JonR]

I tried to smooth out the master cylinder mating surface for the cap. It needs to be very smooth and clean even with a good gasket.

 

[AR67GTX]

Fran, it looks well machined. The old one was mostly rusted in the bowl. The bore looked salvageable. I will have to see if I kept the old one. Between the 66 and 67 I have a storage box of old cores.

JonR - thanks. At least I’m not going crazy. I’m going to see if I can find an o-ring and cut a cork gasket for this cap. Hopefully it will seal up. I’m also wondering if DOT5 will show up under a UV light in the dark. If the cap seal doesn’t work I may look with that.

 

[LowBikeMike]

Clean it up with brake clean, dry, spray with foot powder, look for leak.

 

[retractable]

I have the same issue with my 66 Barracuda and 66 Satelitte. Returned the cyl with the bottom port for the one with no port. I found the the original cap seals much better and I don't fill the bowl as full helps also. h

 

[AR67GTX]

First, here is my next effort considering most likely it’s the cap seal. The MC surface looks smooth so I didn’t do anything to it. I found a rubber flush seal in Plumbing Dept of Lowe’s that requires just a very slight bit of trimming to fit inside the cap. I figure the rubber flapper seal will stay in place better than an o-ring. So installed that over the cardboard seal that was with the cap which I had tacked down to the cap with a thin coat of Permatex#2. And then I put a snug fitting rubber washer under the copper washer on the top bolt. I’ll try to take it out for a short spin tomorrow morning before it gets hot.

If this doesn’t work then I’ll resort to the UV light or some powder and see if I can get a clearer picture of what’s going on.

 

[AR67GTX]

I have the same issue with my 66 Barracuda and 66 Satelitte. Returned the cyl with the bottom port for the one with no port. I found the the original cap seals much better and I don't fill the bowl as full helps also. h

I’ll look to see if I still have the cap for the older (original?) MC.

Thanks

 

[khryslerkid]

First, here is my next effort considering most likely it’s the cap seal. The MC surface looks smooth so I didn’t do anything to it. I found a rubber flush seal in Plumbing Dept of Lowe’s that requires just a very slight bit of trimming to fit inside the cap. I figure the rubber flapper seal will stay in place better than an o-ring. So installed that over the cardboard seal that was with the cap which I had tacked down to the cap with a thin coat of Permatex#2. And then I put a snug fitting rubber washer under the copper washer on the top bolt. I’ll try to take it out for a short spin tomorrow morning before it gets hot.

If this doesn’t work then I’ll resort to the UV light or some powder and see if I can get a clearer picture of what’s going on.

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I see you're using Dot 5.

I've had the same problem with my dual MC 4 drum. It has to be the synthetic that gets past the cap's seal. Just like synthetic motor oil that finds its way past gaskets and seals unlike conventional oils.

I flat sanded the top of the MC with 400 grit and the seal also. Definitely helped but sometimes I'll find a small amount underneath.

Keep an eye on what you are using for your new seal that it's compatible with your fluid.

 

[AR67GTX]

Yeah, I have another one I also picked up which is yellow and advertised as chemically resistant. But it doesn’t have as wide a lip. Thought I would try this one first just to see if this is the problem and maybe an answer. I can also cut one out of .062” cork gasket stock.

When you all sanded your MC lip did you do this while mounted in place? I’m tired of crawling under the dash to remove MC nuts and bolts. Mine feels smooth but I could carefully try it and try not to get anything in the fluid.

 

[JonR]

I was just very slow and very careful smoothing my MC to not get any grit in the fluid - very slow process.

 

[AR67GTX]

I braved the heat to run up and down the boulevard a couple times and parked it. Dry afterwards as usual. See what happens over the next couple of days.

 

[AR67GTX]

So far the MC has stayed dry overnight but I probably won’t declare success until 3 days have passed. I put one of the other flap seals in a cup of DOT5 and I’ll watch it for a few months but I’m a skeptic of all the stories about DOT5 hurting seals. I think the manufacturers are just scared a bunch of people will swap in DOT5 without a complete flushing of the 3/4 out and then have issues - just a CYA move on their part.

 

[MarPar]