My clutch feels fine, soft, fine, soft, etc. The other day I checked the clutch fluid and observed that I was not losing any fluid. I took out the rubber cup and put in back in.

I remember before, the rubber cup had the clutch (brake) fluid in it and there was nothing around it. Now the brake fluid is around the rubber cup and the cup itself is empty.

What is the purpose of this rubber cup? Thanks1999 Ford Ranger Clutch reservoir

  • I have similar issue only with a 1991 chevy S-10 which i believe still has factory clutch master cylinder, my issue is when i add clutch fluid and start to install the top back on reservoir the rubber diaphram inside the cap is thick rubber and does not give any therefore pushes fluid back out, others sites say collapse diaphram with finger before installing top but mine dont collapse, im thinking make a seal for the top to seat then do away with the diaphram Commented Aug 29, 2023 at 15:25

3 Answers 3


The purpose of the rubber cup is to allow for a seal on the master cylinder reservoir, and then to allow for fluid to escape down into the slave cylinder as needed. It allows the fluid to easily change levels while still allowing the reservoir to be sealed (yes, that statement is redundant). It works the same way for the brakes (both use the same hydraulic fluid).

Brake fluid absorbs water at an alarming rate. Keeping it sealed helps keep the fluid fresher and last longer. If you are wondering, I'd consider it a required piece of equipment for the vehicle.

  • Since it is a seal, the fluid should be around it on the outside and not on the inside, correct?
    – wbeard52
    Commented Sep 17, 2016 at 19:34
  • @wbeard52 - Absolutely correct. Commented Sep 17, 2016 at 20:32
  • 2
    @Paulster2, Redundantly speaking we might say that the flexibility of the seal allows the volume of fluid in the reservoir to change without pulling in air (and the moisture that comes with it).
    – dlu
    Commented Sep 18, 2016 at 2:13
  • @dlu - sure ... we could say that ... looks like you did a pretty good job of saying it all on your lonesome! :o) Nice add. Commented Sep 18, 2016 at 2:22

think of the "rubber cup" as a thin film of rubber floating on top of the clutch fluid, and acting as a barrier to prevent ingress of dirt and moisture from the atmosphere entering the fluid. As the fluid level falls due to clutch plate wear and slave cylinder extension, the rubber lowers with the fluid level, still protecting the fluid from contamination. This is its purpose. As clutch / brake fluid is hygroscopic, this action prevents moisture ingress, which not only can cause corrosion but lowers the fluid boiling point, which can lead to boiling of the fluid creating bubbles (which now are compressible) rendering the hydraulic circuit spongy or inoperable when the pedal is depressed.

Cheers Hutch


To keep a seal around so it won't leak

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