Rear Master Cylinder leak

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Hi all,

After meticulously cleaning and rebuilding my rear m/c, I discovered (while bleeding) that there's a brake-fluid leak coming from the hydraulic fitting threads on the back end of the m/c body :cry:

I've tried carefully tightening them, but I've heard the flimsy AL threads can stretch pretty easily ... I'm guessing the PO did this for me already :roll:

Is there a fix for this ??? other than scrapping the m/c. Can I tap some metric oversize threads?? Then what about the line fittings :?:

I have a complete Time Sertreplacement kit (shameless sponsor plug), and have replaced several of the AL threads on my racer (plug holes for ex.) with steel inserts with very satisfactory results. (oops ... I just realized the fittings are a tapered thread -- like a pipe thread -- so a thread insert won't cut it :wink: )

What about Teflon tape?? I might try that tomorrow.

The threads on your rear brake master cylider are straight not tapered.

Check the seal prepartion in the master cylinder where it interfaces with the flare on the end of the brake tubing. If corrosion or spooge is evident, this could be your problem. Also check the flare on the tubing for damage. Frankly, I would replace this small sectioin of tubing for the sake of good order and a leak-free connection.

I would not use teflon tape to cure your leak. If you clean-up the master cylinder seal area and replace the tubing your leak will be fixed; it's that simple!


'75 Norton Commando; '80 Schwinn Super Le Tour; '93 Peugeot; '90 HD Electra Glide; '99 C5
Thanks Jason.

I'm going to order it when I finish this post.

I'm thinking that when I cleaned the pipe and on my wire wheel, I may have left some fine scratches on the flare end which, even when seated, are allowing some fluid to seep by.

I'll also get in and buff the female portion of the seat :shock: in case there's an uneven or scratched area.
Hi Jason et al,

I just pulled it all apart (my replacement pipe piece is on its way). I noticed the shiny mating marks on the flared head only went around 75% of its circumference. That told me it wasn't seating over the entire circle, and that's where the fluid was leaking by.

I was able to see down into the female surface where it seats, and saw there was indeed some pitting (I had cleaned this area previously with a dremel, but didn't inspect with my jewlers loupe).

The pitting looks to be about .020" +/- deep, However it looks like if there was a way to cut a new seat, there's plenty of metal available.

Question: is there such a seat cutting tool???? If not, I'll make one and give it a shot ... got nothing to lose. Maybe a drill bit with the shank turned down to just slip inside the threaded nut which presses the surfaces together. If I screw the piece in, the nut will keep it centered, and the bit head will be free to move in/out and cut a new surface. The profile won't match, but the angle should be close enough to provide a clean mating surface.

Cheaper than a new m/c eh 8)

I'll let you know how it works :arrow: :arrow:
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