I understand that when you overfill some models of Shimano hydraulic brakes with oil, the excess will flow out of a one-way valve called weeping port, but cannot find any technical documents, pictures or videos that point out where this valve actually is. I suspect it is the conspicuous, but rarely mentioned, cannon-resembling big round ending on top of many Shimano brake lever models, which I highlight in the picture below.

enter image description here

The reason I am asking is because when I pushed back my caliper pistons to install new brake pads today, a lot of oil suddenly squirted from this part of the lever. The lever became very soft of course, so I bled it and it felt fine, but the oil continued to seep (or, well, weep) slightly as I rode the bike later.

So, is this indeed the weeping port and how exactly should it operate? Does it just let the excess oil out after which the brake functions normally - as mine does so far, but with oil still flowing out very slightly - or should it be repaired?


From my understanding, that port is a pressure compensation port. When the oil level changes because of pad wear, temperature, or a leak, air will enter through that port to balance the reservoir’s pressure. The oil is kept sealed by the membrane, which you might have torn due to overfilling.

There’s a small port in the side of the master cylinder that connects the reservoir to the cylinder. When you actuate the lever, the piston travels past the port, allowing the system to build up pressure. When you release the lever, the port opens up again and does any necessary compensation.

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    In the meantime I have learnt that what you wrote is more or less correct. I have also found out that this sort of leak happens quite often. There used to be a solution, which I cannot apply to my brakes, I assume because the Shimano brake design has changed slightly, but I am posting the link to it because it should be applicable to somewhat older models: mtbr.com/threads/2011-xtr-m985-brakes-leaking.750869/… – Mick Jan 23 at 15:33
  • Cycling tech is awesome! Nice find. – love2phish Jan 23 at 15:46
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    Update to first comment: The solution I linked to actually still works for newest Shimano brakes! Just go to the second page of the linked-to thread to see the photos! I haven't checked that thread for a while so I have not seen them until today. The guy who posted them actually had a punctured diaphragm so he had to replace it, but the photos illustrate the procedure to remove the cap and the diaphragm in order to just reset it. – Mick Jan 23 at 15:58

There is a membrane behind the black plastic end cap and that membrane can be damaged for instance when too much oil is put into the system. Since the membrane is a part that cannot be damaged just by using the brakes as intended or bled the right way, the plastic end cap cannot be removed and therefore the membrane can't be replaced. This is probably why your brakes are not working.

enter image description here

Hope this helps!

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    interesting, is the membrane covered hole there just from machining the part? – Swifty Jan 22 at 21:15
  • The hole is normal and should be found on all models of this type. Obviously it should not leak a lot. I can only assume the hole is strategic so that if one was to overfill, the extra would just flow out the extra hole. Don't take my word for granted though, still have some research to do! – love2phish Jan 22 at 21:24
  • Found this: patents.google.com/patent/US20180056967A1/en I have looked through it and kind-of/sort-of gives you an idea what could go wrong. – love2phish Jan 22 at 21:37
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    seems to be the patent for SRAM's version of Shimano's J-kit where the lever and hose can be shipped pre-bled but disconnected. The mention of a 'port' I read was to to with a port valve at the interface of hose and lever where it is closed until the two are mated when it opens automagically. but I didn't read it all and don't have the answer, I'm intrigued to know – Swifty Jan 23 at 13:41
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    I have found out that the cap actually could be removed and diaphragm reset on older models of Shimano MTB brakes, but it does not seem to be the case with the newest models. I gave the link to this info in the comment to MaplePanda's answer. By the way, my brake works just fine despite the leak. The leak is just a squirt that happens when I overfill the brake during bleeding, and does not occur later, during use. However I can imagine that it could be a more serious problem for some people, if the membrane was damaged or if it moved more than mine. – Mick Jan 23 at 15:37

From my knowledge with MTB Brakes there should not be oil coming out anywhere, as braking relies on putting pressure into the lines.

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