Topic: Leaking Master Cylinder Cap

[nomadwarmachine]

After rebuilding my master cylinder (and adding stainless lines) a little while ago, I have recently developed a problem with fluid leaking out from under the master cylinder cap.  This appears to be associated with the vibrations produced by high RPMS -- at 7,000 it is practically streaming out from under the cap.  This happens regardless of how hard I screw down the cap.

I am going to order a new rubber diaphragm, since my current one has some tiny cracks, but I would be surprised if this is the cause.  Is it possible that I am putting the cap on wrong?  I always fill to the fill line, then place the black diaphragm on top, then screw on the cap tightly.  Any help or past experience is appreciated!!

[shoemanII]

is the front brake dragging more than is usual?  this may heat up the brake unit/fluid, causing some expansion of the fluid.  or maybe try taking some fluid out, say 1/4inch below the "fill" line.  don't really think we need a reservoir of that volumn anyway.............

good luck on this, sounds like you're still riding.  lucky you, just pulled the cylinder today for new top end gaskets/ "o"-rings.
bobp

[TwoTired]

Cracks in the diaphragm WILL leak. The diaphragm is there to allow the fluid the expand and contract with temperature and allow for changes in reservoir volume.  The cap itself is vented to the atmosphere to allow the diaphragm to move. The cap only applies pressure to the diaphragm around the periphery for a seal.  That is the cap's only responsibility for fluid containment.

Cheers,

[Bob Wessner]

There should also be a nylon washer-like part, Honda calls it the diaphram plate. This fits between the cap itself and the rubber diaphram. I'm guessing it prevent crimping the diaphram lip as you tighten the cap. Without it, crimps form which could also cause leaks.

[nomadwarmachine]

Thanks everybody.  I will place an order for a new diaphragm tomorrow morning!  Brakes are not dragging, so I doubt that the system is running too hot -- it seems to be most closely tied to engine rpm, and vibration, so I think that the fluid must be seeping through the cracks in the diaphragm and around the circumference of cap.

[Jay B]

Cracks in the diaphragm WILL leak. The diaphragm is there to allow the fluid the expand and contract with temperature and allow for changes in reservoir volume.  The cap itself is vented to the atmosphere to allow the diaphragm to move. The cap only applies pressure to the diaphragm around the periphery for a seal.  That is the cap's only responsibility for fluid containment.

Cheers,

TT- I beleive the diaphram is also there to separate the fluid from the atmosphere, reducing or eliminating moisture in the fluid from humidity.

[TwoTired]

Cracks in the diaphragm WILL leak. The diaphragm is there to allow the fluid the expand and contract with temperature and allow for changes in reservoir volume.  The cap itself is vented to the atmosphere to allow the diaphragm to move. The cap only applies pressure to the diaphragm around the periphery for a seal.  That is the cap's only responsibility for fluid containment.

Cheers,

TT- I beleive the diaphram is also there to separate the fluid from the atmosphere, reducing or eliminating moisture in the fluid from humidity.

Of course.  But, a simple gasket would do that.  A diaphragm is needed for the reasons stated.

BTW, the membrane isn't a total isolation.  Research osmosis.  Water can still enter the reservior, albeit slowly.

Cheers,