Hi everyone, this is my first post and I’m hoping someone here can help because I’m honestly out of ideas.

I rebuilt my Tilton throwout bearing and put everything back together. We pressure bled it, pedal bled it, set the clutch to about 1/4 inch past disengagement, and everything looked totally normal.

The weird stuff started after the first time we started the engine. Right after that, the pedal suddenly lost pressure. and some weird stuff started happening with the clutch system. and continues with the engine off, so we don’t actually know if the engine caused it. It just started right after that first start and now it’s fully repeatable.

When it happens, the pedal hisses at the top and the engagement point drops way down and the system loses most of its pressure. Then I need to pump it back up. It does not act like a leak at all. It feels almost like something is building too much pressure or pulling a hydraulic vacuum at the very top. Once it “pops,” the pedal goes soft and the throwout moves back to its normal position.

One thing I noticed is that this does NOT happen if I release the clutch quickly. I’m assuming whatever is happening internally doesn’t have enough time to kick in unless I’m slowly coming up to the very top of the stroke.

You can pump the pedal up again and everything feels perfect until you hit the very top again and it hisses.

The confusing part is that this did not happen even once during the throwout calibration. We pressed the pedal at the top, middle, and bottom probably over 10 times. Everything felt perfect until we started the engine once, and since then it happens every single time whether the engine is running or not.

We installed a brand new Wilwood master cylinder just to rule it out and the exact same thing happens. The hissing sound is right where the master pulls in new fluid at the top of the stroke.

We’ve been chasing this for days and still have no idea what’s happening.

Here are the videos:

1. Right after bleeding, everything works normally (the squeak is just new seals)
   
   You Tube
2. Throwout behavior after the hissing
   It starts extended about 1/3 to 1/2 further than normal, then collapses back when it hisses.
   
   You Tube
3. Pedal position during the hissing event
   The pedal is firm, then suddenly hisses and shakes.
   
   You Tube
4. Master cylinder reservoir view
   No air bubbles coming in from the top.
   
   You Tube

If anything sounds unclear or you want more info or videos, I can explain anything. This has been a super weird issue and I’m hoping someone has seen something similar.

Thanks.

 

I'm at work so I can't hear the sound on a video, but it seems to me your master cylinder likely has a seal leaking internally. The sound may very well be the fluid escaping through a small tear in the seal ( which is why a fast application doesn't affect it, no time for the fluid to leak ).

 

Thanks for the reply. I actually figured out the issue yesterday, but it required pulling the trans back out.

I should have updated this earlier because it ended up being a pretty interesting one. As I mentioned in the original post, I rebuilt my Tilton throwout bearing. When you rebuild it, there are two seals that need to be installed. The main pressure seal is a single unit that has the main sealing lip with a sneaky inner O-ring that sits inside that seal and a scraper seal.
On the bottom of the Tilton TOB there are two holes where the fluid transfers through. The problem wasn’t a “leak” in the normal sense. What was happening was the inner O-ring inside the main seal was separating from the sealing ring. It wasn’t leaking fluid out, but it was partially blocking the high-pressure passage inside the throwout bearing.

That blockage is what caused all the weird symptoms. The high-pressure fluid couldn’t return back to the master cylinder, so the whole system would basically “ratchet” and hold pressure in the wrong spot. When I would pump the pedal, the extra hydraulic pressure inside the TOB would push the loose O-ring out of the way for a moment, letting the high-pressure fluid equalize again. Then it would randomly get stuck again on the next slow release.

This also explains why if I released the pedal quickly, it never made the hissing sound. When I moved my foot quickly, the O-ring basically got flung back into place instantly and sealed the passage before fluid had time to move. But when I released the pedal slowly, the fluid was able to start flowing back, and the loose O-ring would shift and clog the hole at the exact wrong moment, causing all the pressure weirdness and the hissing.

This all came down to me not installing the seal with the proper tool. I re-did the rebuild yesterday using the correct Tilton seal installation tool and installed a brand-new seal. As soon as we put everything back together, all the issues were gone.

It was a total “chase your tail” problem because all the noise and symptoms seemed to be coming from the master cylinder, but that was only because the high-pressure fluid was suddenly equalizing after being jammed inside the throwout bearing.

Everything is working perfect now.

 

Great, glad to hear it. Thanks for coming back to post your solution, as someone else may have this problem someday and being able to search the site and find answers is what makes it so valuable.