Ideas? Sticking clutch pedal only when braking heavily

[Oldcarguy]

I know a sticking (on return) clutch pedal can be a symptom for slave going bad, air in lines, clutch bearing, etc. but I’ve got a slightly different manifestation.

Under normal and spirited (heel and toe) driving I have no clutch pedal issues. Smooth engagement, proper pedal travel, etc. Clutch has ~4,000 miles on it since rebuild by PO’s independent shop.

However, when braking extremely hard with the clutch disengaged simultaneously (classic ‘panic stop’) the clutch pedal returns halfway and sticks. If I depress it again, it returns normally. I discovered this because I occasionally engage the abs on purpose to exercise it. But sometimes the pedal will also stick under very heavy braking without getting into the abs. Brake and clutch pedals are firm.

I’m going to purge the brake/clutch fluid (with Motul 600) and see where I’m at. If nothing changes regarding the sticking pedal, looks like I may be headed for a clutch slave cylinder replacement next. I need this fixed before I put the car on the track for HPDE events.

Any thoughts?

[Gilles]

Why don't you try to cleaning (and lube) the shaft where the pedal hangs from and the return spring as well?

[Oldcarguy]

Originally Posted by Gilles

Why don't you try to cleaning the shaft where the pedal hangs from and the return spring as well?

Will do, it’s creaking a little. I’m also going to see what the status of my helper / return spring is. It’s still there but the pedal is noticeably more firm than my friend’s 986. The spring may be broken.

[Blackroot]

Originally Posted by Oldcarguy

I know a sticking (on return) clutch pedal can be a symptom for slave going bad, air in lines, clutch bearing, etc. but I’ve got a slightly different manifestation.

Under normal and spirited (heel and toe) driving I have no clutch pedal issues. Smooth engagement, proper pedal travel, etc. Clutch has ~4,000 miles on it since rebuild by PO’s independent shop.

However, when braking extremely hard with the clutch disengaged simultaneously (classic ‘panic stop’) the clutch pedal returns halfway and sticks. If I depress it again, it returns normally. I discovered this because I occasionally engage the abs on purpose to exercise it. But sometimes the pedal will also stick under very heavy braking without getting into the abs. Brake and clutch pedals are firm.

I’m going to purge the brake/clutch fluid (with Motul 600) and see where I’m at. If nothing changes regarding the sticking pedal, looks like I may be headed for a clutch slave cylinder replacement next. I need this fixed before I put the car on the track for HPDE events.

Any thoughts?

The brake fluid brand/type would be interesting if that changes.
DOT 4+ is DOT4 +
The volume isn't going to change in the master or slave with new fluid.
Might want to see if the slave cylinder hard line (12mm flare wrench used only) is tight. The slave bleeder is 11mm.
Could be many things...
1. pressure plate springs are weak or failing.
2. slave cylinder is not quite right with air bubbles.
3. Bad throw out bearing or in pieces
4. Master cylinder return spring (mentioned)

Also, if you think about what you just said "when braking extremely hard with the clutch disengaged simultaneously (classic ‘panic stop’) the clutch pedal returns halfway and sticks", remember these are shared fluid systems, so the caliper might not be returning enough fluid to pull up the clutch.
Seems to point to air in the system

[Oldcarguy]

Originally Posted by Blackroot

The brake fluid brand/type would be interesting if that changes.
DOT 4+ is DOT4 +
The volume isn't going to change in the master or slave with new fluid.
Might want to see if the hard line (12mm flare wrench used only) is tight. The bleeder is 11mm.
Could be many things...
1. pressure plate springs are weak or failing.
2. slave cylinder is not quite right with air bubbles.
3. Bad throw out bearing or in pieces
4. Master cylinder return spring (mentioned)

Thanks BR. Hopefully it’s air bubbles that the fluid change will take care of, but one or more of the points you mention are definitely in the running.

The thing that gets me is that it only does it in conjunction with extreme braking.

According to the paperwork I received from the PO, the clutch only has ~4,000 miles on it. But, I do have a heavy clutch pedal even though the helper spring *appears* to be in good shape & working properly, the engagement point is good, no chattering, no leakage, no gear grinding etc. If a check of the helper spring and the fluid purge with my Motive doesn’t fix the issue I may kick it over to my indie. Too many other irons in the fire and no time to pull the transaxle and redo the clutch before first HPDE in April.

Frankly I am leaning toward a hinky throw out bearing or pressure plate but I don’t get why it only happens in conjunction with extreme braking. Front motor mount is relatively new (987), but maybe a transaxle mount?? Clutch is hydraulic and the transaxle/clutch assembly is a pretty solid unit so I just don’t see a possibility there.

[Blackroot]

Originally Posted by Oldcarguy

Thanks BR. Hopefully it’s air bubbles that the fluid change will take care of, but one or more of the points you mention are definitely in the running.

The thing that gets me is that it only does it in conjunction with extreme braking.

According to the paperwork I received from the PO, the clutch only has ~4,000 miles on it. But, I do have a heavy clutch pedal even though the helper spring *appears* to be in good shape & working properly, the engagement point is good, no chattering, no leakage, no gear grinding etc. If a check of the helper spring and the fluid purge with my Motive doesn’t fix the issue I may kick it over to my indie. Too many other irons in the fire and no time to pull the transaxle and redo the clutch before first HPDE in April.

Frankly I am leaning toward a hinky throw out bearing or pressure plate but I don’t get why it only happens in conjunction with extreme braking. Front motor mount is relatively new (987), but maybe a transaxle mount?? Clutch is hydraulic and the transaxle/clutch assembly is a pretty solid unit so I just don’t see a possibility there.

Yup and as I said on the bottom here-
"Also, if you think about what you just said "when braking extremely hard with the clutch disengaged simultaneously (classic ‘panic stop’) the clutch pedal returns halfway and sticks", remember these are shared fluid systems, so the caliper might not be returning enough fluid to pull up the clutch.
Seems to point to air in the system"
If I was to bet, I would say air

[Oldcarguy]

Originally Posted by Blackroot

Yup and as I said on the bottom here-
"Also, if you think about what you just said "when braking extremely hard with the clutch disengaged simultaneously (classic ‘panic stop’) the clutch pedal returns halfway and sticks", remember these are shared fluid systems, so the caliper might not be returning enough fluid to pull up the clutch.
Seems to point to air in the system"
If I was to bet, I would say air

I’m on my iPad and I thought I read your whole post, but I missed your very pertinent point at the bottom. Those 4 pot calipers are holding a lot of fluid under extreme braking. Also, I have no records indicating that the fluid has been changed in the last few years so there may be who knows what in the system.

I’ll try to change the fluid and purge the brake and clutch systems (also ensuring to engage PSAM and ABS) next week. I’ll update as soon as I have the result.

[Blackroot]

Originally Posted by Oldcarguy

I’m on my iPad and I thought I read your whole post, but I missed your very pertinent point at the bottom. Those 4 pot calipers are holding a lot of fluid under extreme braking. Also, I have no records indicating that the fluid has been changed in the last few years so there may be who knows what in the system.

I’ll try to change the fluid and purge the brake and clutch systems next week. I’ll update as soon as I have the result.

I just used one of these with fantastic results. I did it dry @ 22PSI
https://www.amazon.com/Motive-Products-European-Bleeder-Pressure/dp/B0002KM5L0

Be quick on the bleeder or you will drain the brake fluid tank.
I'm a weekend warrior, like you

[Oldcarguy]

Originally Posted by Blackroot

I just used one of these with fantastic results. I did it dry @ 22PSI
https://www.amazon.com/Motive-Products-European-Bleeder-Pressure/dp/B0002KM5L0

Be quick on the bleeder or you will drain the brake fluid tank

My Motive just came in. I’ll have my shop assistant (wife) monitoring to ensure I don’t introduce new air into the system.

[Blackroot]

Originally Posted by Oldcarguy

My Motive just came in. I’ll have my shop assistant (wife) monitoring to ensure I don’t introduce new air into the system.

You can either use the bleeder screw on the slave cylinder or a brake caliper.
I would start with slave.
11mm socket, and be ready to get soaked unless you attached a hose to the bleeder valve to a tray.
Brake fluid actually doesn't taste bad, but don't swallow
Tight quarters.
Make sure the brake fluid tank is completely full
Also, start with clutch fully up, pump the bleeder tank to 22psi. get under car and open bleeder.
When air noise stops and a steady stream suddenly appears, tighten valve.
Get in car and try clutch pedal (fill the brake fluid tank first as it will have lowered)
Might be perfect right there.
If not, repeat

[Oldcarguy]

Originally Posted by Blackroot

You can either use the bleeder screw on the slave cylinder or a brake caliper.
I would start with slave.
11mm socket, and be ready to get soaked unless you attached a hose to the bleeder valve to a tray.
Brake fluid actually doesn't taste bad, but don't swallow

Duly noted

[Blackroot]

Originally Posted by Oldcarguy

Duly noted

I edited below, so here it was

Make sure the brake fluid tank is completely full
Also, start with clutch fully up, pump the bleeder tank to 22psi. get under car and open bleeder.
When air noise stops and a steady stream suddenly appears, tighten valve.
Get in car and try clutch pedal (fill the brake fluid tank first as it will have lowered)
Might be perfect right there.
If not, repeat

[Blackroot]

Originally Posted by Blackroot

I edited below, so here it was

Make sure the brake fluid tank is completely full
Also, start with clutch fully up, pump the bleeder tank to 22psi. get under car and open bleeder.
When air noise stops and a steady stream suddenly appears, tighten valve.
Get in car and try clutch pedal (fill the brake fluid tank first as it will have lowered)
Might be perfect right there.
If not, repeat

I tried the Pelican way with starting the bleed process with clutch pedal down and never had any luck, BTW.
Nothing against Pelican as they are tremendously valuable, and I source them for almost everything I do.
Up always worked for me.

[Geof3]

Originally Posted by Oldcarguy

I’m on my iPad and I thought I read your whole post, but I missed your very pertinent point at the bottom. Those 4 pot calipers are holding a lot of fluid under extreme braking. Also, I have no records indicating that the fluid has been changed in the last few years so there may be who knows what in the system.

I’ll try to change the fluid and purge the brake and clutch systems (also ensuring to engage PSAM and ABS) next week. I’ll update as soon as I have the result.

HPDE events in my region require brake fluid replacement annually. Still seems like an odd symptom overall. Don’t forget to bleed your clutch slave as well...

[Oldcarguy]

Originally Posted by Geof3

HPDE events in my region require brake fluid replacement annually. Still seems like an odd symptom overall. Don’t forget to bleed your clutch slave as well...

Yes, our region also requires annual brake fluid replacement. I’ll definitely bleed the clutch. It is a strange symptom, first time I’ve run across something like this. It’ll be interesting to find the root cause.

[Oldcarguy]

Originally Posted by Blackroot

I tried the Pelican way with starting the bleed process with clutch pedal down and never had any luck, BTW.
Nothing against Pelican as they are tremendously valuable, and I source them for almost everything I do.
Up always worked for me.

Thanks. I’ve also read and seen videos doing the clutch bleed both ways. Because I’ll be doing a pressure bleed, pedal up seems most intuitive to me also.

[Blackroot]

Originally Posted by Oldcarguy

Thanks. I’ve also read and seen videos doing the clutch bleed both ways. Because I’ll be doing a pressure bleed, pedal up seems most intuitive to me also.

Hello
Any update ?

[JayG]

Unless I am mistaken ( I certainly could be) you must bleed the clutch with the pedal down
see: Clutch Bleeding With a Power Bleeder

[Blackroot]

Originally Posted by JayG

Unless I am mistaken ( I certainly could be) you must bleed the clutch with the pedal down
see: Clutch Bleeding With a Power Bleeder

You are not mistaken, but I tried the same thing 2x.
Both times failed miserably and I lost 16$ worth of brake fluid.
I found success with pedal up.
I have driven the car 400 miles since all the work and it shifts like it just came out of the factory.
Butter.
Again, pedal up worked for me, and doesn't mean the best way.
Just to be clear, I have a 2004 S special, and I had a new slave cylinder that I installed on the gear box before it went back in.
Also, be sure to drain and re-fill your gearbox while you have it out. 75W Mobile 1

BTW, there is nothing more rewarding than driving your car after all that hard work.
Faster/better or not, it was worth every minute of work.
In fact, next time I can do it in half the time from this experience.

[Oldcarguy]

Originally Posted by Blackroot

Hello

Any update ?

Ran smack into the Holidays and they stopped my 986 work like running into a wall. Got back to it today, lifted the car and discovered I have a torn left CV transaxle boot. Must have just happened. Replaced the front plastic “air guides” that keep the radiator heat from the front brakes and called it a day with a refreshing adult beverage.

I’ll update as soon as I have her back on the ground, but it will be a while (.5 quart sump extension, oil pump extension, sump baffle, underbody plastic repair, CV boot replacement, brake fluid purge and bleed).






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