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Leaking 40 Special Master Cylinder


Podster

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I just had the master cylinder for my 40 Special  bored and sleeved. I decided to buy a rebuild kit and do the rebuild myself. Piece of cake thought, except for one problem . It leaks from the copper seal ring between the master cylinder body  and the treaded port base. I torqued the heck out of the threaded base with a pair of Channel Locks but it still leaks. Is there some kind of trick to sealing this area up, like Teflon tape on the threads? 

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Guest gmorse

I had the same problem on my '39 at least 10 years ago. If my memory is correct, after I took it apart, and used a straight-edge on the casting, there was a very small defect in the casting. Did not leak before, so the tiny area must have been filled with rust, or some other junk. Again, if my memory is correct, I had the casting very slightly machined to be completely flat. I have also heard of the fitting that screws into the cylinder being not flat. I did not have that problem, just the old casting was not flat enough to fully seal.

Also, closely examine the brass washer for any defects which could cause the same problem.

Gary

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First, the sealing surfaces must be smooth. Then you have to fully anneal the copper seal ring.

You anneal the ring by heating it until it is fully red hot and then quench it in water. Taught to me by an old pipe fitter.

Some say that you do not have to quench it, but I always do and it always works

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Thanks for responding. All your responses gave me some information to work with. I considered having the machined seating area kissed, and I may do that. However I noticed that the wall thickness at the bottom end of the bore diameter  is no more than 1/16" thick while the top end is about 1/4 " thick. That is not much of a sealing surface on the bottom. It is not a very good sand casting and may have shrink in that area.

I also compared the old copper seal ring to the one I received in the repair kit. The old one was .066" thick the new one is .039" thick. I good reason to buy NOS.They are both scratched up from assembly and disassembly. Anyone know where I could buy copper seal rings?

Billy, when you loosened and tightened it 100 times, was it  a quarter turn back and forth each time? Sounds like you made a groove in the sealing ring which increased the sealing surface. Don's idea about annealing the ring does the same thing except the soft ring is being squished. Did you use an Oxy-Acetylene torch? The ring didn't warp?

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You just need a propane torch to get the copper to red hot.  (you can even di it with a cigarette lighter)

You do not need to worry if the ring gets distorted. Since it becomes dead soft, the process of tightening will flatten it out.

The idea behind the annealing is to make it very soft so it will conform to any irregularities on the sealing surfaces. However, the smoother the better.

 

You do not have to worry about the copper getting to soft and blowing out. It still has plenty of strength

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