TR2/3/3A Clutch Swallows Air -- TR3

Today, I present my baffling and annoying problem.

Air gets into my clutch hydraulics even though FLUID DOES NOT LEAK OUT.

The specifics:
1) Bubbles are in the slave cylinder, which is to say, low in the system, not up in the master. Three pumps on the pedal while bleeding and they are gone.
2) Sometimes this happens after 3 months. Sometimes only 2 weeks between bleeds.
3) The fluid level in my reservoir goes UP -- displaced by the air that somehow gets in. There is NO indication of fluid loss. Only air ingress.
4) It seems to happen overnight -- as in -- the car was driving just peachy when garaged for the night. I get in to start it in the morning and my clutch is WAY down from the last time I pushed it.
5) Sometimes it happens just a little bit. I notice the clutch pedal is a <span style="font-style: italic">little</span> lower -- and I can pump it up -- but it's not so bad I have to bleed it right now. It may stay that way for weeks and then, bango, see item 4.

I can think of no mechanism whereby this should occur. Therefore I am at a loss as to what to fix. Last year, I rebuilt the master, hoping to cure this. I am about to R & R the slave, but I don't know why. I'm just swatting at things that <span style="font-style: italic">could</span> be the culprit, without any real treatment plan.

Comments? Suggestions? Excorcism will not be considered without a sound theological discourse.

How's the return spring on the slave? Have you tried pushing the pushrod back into the cylinder by hand?

The slave spring should always immediately return the slave piston against the internal stop, but if the spring is weak (or someone has tried to substitute an old screen door spring or whatever), it may not always return the piston. If the spring inside the MC returns the master piston faster than the slave piston, it might pull somewhat negative pressure on the hydraulics, which might pull air past a worn slave seal.

Not likely, but the only thing I can think of offhand.

Moseso said:

I am about to R & R the slave, but I don't know why.

Click to expand...

That's what I'd do, for what it's worth.

Tom

You Might not be experiencing air, but rather the plunger just going right down without generating any pressure and not leaking yet. I would try a new and different master cylinder.

I'm with you, Randall. Negative pressure inside the slave could easily draw a little air past the rubber seal. Sadly, I have a perfectly fine spring, drawing the clutch lever back into the slave -- not to mention that the clutch cover springs would <span style="font-style: italic">certainly</span> prevent negative pressure inside the slave. My rod-to-slave freeplay is negligible -- almost bottomed.

This one's got me buffaloed. I've been rebuilding hydraulic car parts since 1969. In all that time, I've never seen these symptoms. Where's my sound theological argument -- so I can decide to perform an exorcism? Short of that, where's my negative pressure in the system?

sp53 said:

You Might not be experiencing air, but rather the plunger just going right down without generating any pressure and not leaking yet. I would try a new and different master cylinder.

Click to expand...

I am definitely experiencing air. Bubbles definitely bleed out when I decide that the car won't run without another clutch bleed. Then it works right (for a while) again.

I'm a game as the next guy for a good exorcism, but wouldn't a seal on the way out display very similar symptoms?

I see some Master Cylinders with bolts at the end to limit the return of the MC piston and/or pedal travel. Is it possible yours are too far out, allowing the piston to travel so far back that it allows air in, without letting fluid out? Seems unlikely.

I like Randall's assessment of the situation, even if the spring is OK.

The seal in the slave cylinder is designed to seal against high pressure in the cylinder. If the pressure inside is below that of the atmosphere, it's easy for air to leak past the seal into the cylinder. I've had this happen with my Bugeye Sprite.

Even if everything appears to be OK, it's possible that somehow, sometime, you might have a situation where the return of the piston in the slave is sluggish, and you get a little air into it past the seal. This situation may not present itself easily or often, but you stated that it happens only occasionally, so I think it all makes sense.

I'd rebuild the slave cylinder. When you do that, check carefully for pitting, and hone it out well anyway. Even though I've worked with seals a lot (high vacuum, not in the zoo), I'm continually amazed at how a seemingly perfect surface and new seal still can leak. If it's not perfect, you're all that much worse off.

Check the vent on the reservoir. Fluid gets hot and expands. Fluid cools and contracts, making a vacuum and pulling air in. If it can't come in through the vent...it'll find another way.

John

John --
I like this idea. It goes well with the "happens overnight" symptom.
A simple fix, too, if it's the issue.
If it is the issue, thank god it isn't happening to the brakes too!

Will report on findings...

CJD said:

Check the vent on the reservoir. Fluid gets hot and expands. Fluid cools and contracts, making a vacuum and pulling air in. If it can't come in through the vent...it'll find another way.

Click to expand...

Good thought!

If the vent is OK, you might double-check that the pedal is returning fully every time. If the MC piston were remaining just slightly depressed sometimes (enough for the 'foot' valve to stay closed), you could possibly get the same effect.

Just had the same problem. I had just driven 1500 miles to Provost, AB' The next day the clutch went to the floor with out disengaging. My cousin put it up on his hoist and we found much air in the cylinder. Only having domestic car and truck parts we rebuilt the slave and bled it. It worked great so we though it was fixed. That evening we went to the Bunnock championship matches at Macklin, SK. When we were leaving the clutch had the same symptoms.

So we decided to make the drive from Provost to Calgary with no clutch to connect with Dave More who is also a TR6 driver. And along with his neighbor Shawn, have a considerable catch of TR parts. A different master cylinder did the trick. Tonight we are in Jasper, AB after a beautiful 7 hour drive in the Rockies.

My slave and master were both less than a year old. GIGO.

I pulled the cap on the reservoir and took a look. I had been unable to find a proper sized gasket for the top, when I was putting things together, but I found a generic brake reservoir cap rubber gasket at the FLAPS. I trimmed it to fit the TR cap, and poked a hole in the center of it. It is not a ring, it's a disc. Looking at this arrangement, I decided it is conceivable that it <span style="font-style: italic">could</span> have sealed the cap vent, on occasion. So I cut the center out of it, totally clearing the vent in the metal cap.

<span style="font-style: italic">MAYBE</span> this will prevent recurrence of my weird-ass clutch mystery. Time will tell. I'll burp it again, just to be sure I'm getting a good fresh start, and see what happens. If you never hear me complain about this again, that will mean "good news."

Well, that will fix the air sucking, but now you will get brake fluid out of the vent hole. The way this arrangement works is that the rubber gasket acts as a bellows. It moves to allow pressure in and out, but it is supposed to totally seal the fluid in...no holes at all. The vent that needs checking is the one in the metal cap. It allows air to the top of the bellows gasket to equalize the pressure in the system as the fluid expands and contracts.

John

John, I think Moses' point was that his generic seal is flat, and blocked the original vent through the metal cap. The original TR3 reservoir (and even later TRs as I recall) did not use the bellows arrangement that you are talking about.

If you or anyone knows of a bellows seal that will fit inside the TR3 reservoir cap, I would like to know about it. Strikes me as a very worthwhile modification, not visible to a judge (not that I care about judges).

TR3driver said:

...If you or anyone knows of a bellows seal that will fit inside the TR3 reservoir cap, I would like to know about it. Strikes me as a very worthwhile modification, not visible to a judge (not that I care about judges).

Click to expand...

One of the local Healey guys (same reservoir as the TR3A I think) came up with a GM seal from the Help! section of the FLAPS.

Possibly this one: https://www.dormanproducts.com/p-21904-42104.aspx



Can't guarantee a fit as I held one in my hand but didn't get to actually try it on the TR.

Cool, Thanks! I'll see if I can pick one up.

That looks a lot like what I have. What I had before was a fiber ring that sealed the cap to the edge of the reservoir -- but was open in the center. The TR cap appears to hope to contain the fluid by a "labyrinth" of sorts. If the res is filled just to the top of the center tube, I get no fluid escaping through the vent.

Sorry, my seal from Moss was a bellows, so I assumed this is the way it was supposed to be. It looks like the pic above. An interesting point...every time I remove my cap the bellows is sucked down towards the reservoir.

John