Bizarre clutch behavior

LiquidMoose

Well-known member
On another road trip.

Perfect clutch feel went departing on this trip, after a couple of hours of freeway driving I’ll do a downshift on an exit ramp, clutch pedal goes to floor. This has happened a few times in the last few days. I’ll gravity bleed at the slave with pedal up, then bleed again with the pedal down, I may have to do this a few times but I’ve been successful so far, get a nice high pedal. I’ll then drive on surface streets, lots of traffic lights, stops signs, slow speeds, a lot of clutching, I did this for three days, no problem, same nice high pedal. Got back on the highway today and after a couple of hours of driving 65-75 mph, (not using the clutch of course), made an exit and no clutch at all. Thinking it is heat, I probed the slave and lines, read 180*. Did my usual bleeding routine, no dice.

No fluid loss.

Before trip:

New Dorman clutch master, thinking the old Dorman was bad, but seals looked good.

Pulled slave, seals looked good.

Installed new rubber hose between the firewall and slave.

Simply put, the clutch is great around town, after a couple of hours of freeway driving, pedal goes to floor.

I am completely baffled.
 

javelinadave

Administrator
Staff member
Sounds like something is heating up and loosing its seal. Perhaps fab up a sheet metal heat shield to keep the slave and the piping cooler. When I had my 200Tdi and an LT77 the fluid changed colors pretty quickly from the exhaust pipe heat but I never experienced anything like what you are seeing.
 

LiquidMoose

Well-known member
Thanks guys.
I installed a shield, after an earlier episode, that covered the slave and the S pipe, I used a piece of Dynamat hood heat shield, 3/4" foam foil wrapped fixed with zip ties. I was surprised it didn't work. Grrr...
Probing the slave and hoses with an IR temp gun as soon as I pulled over, I read 180F. That's not hot.
If there is air in the lines how can I have a nice high pedal driving around Tucson for an hour then after 2 hours at freeway speeds I have no pedal at all? A diagnostician I am not.
 

rocky

NAS-ROW Addict
This is so weird. To the point that I'm wondering if your brake fluid has been contaminated and lost its properties.
 

LiquidMoose

Well-known member
Only 500 miles from home!
My routine is to flush the system with fresh fluid every morning which gives a nice high pedal. I've kept my highway speed lower which seems to help; I get a low pedal after about an hour drive but I can still change gears. I also added more insulation around slave and hoses.
Just weird.
 

LiquidMoose

Well-known member
I completed another 2000 mile trip after making repairs/mods: no clutch issues.

I replaced the clutch slave. The seals in the old one “looked” fine, the bell housing was bone dry of hydraulic fluid, therefore no leaks from the slave.

I installed a 4 AN braided SS line direct from the clutch master to the slave eliminating the rubber flex line and the metal curlicue pipe from the flex line to the slave. Bleeding was still a challenge but reverse bleeding from bottom to top with the oil can method worked. The pedal was significantly firmer (as compared to the 2 years that I've had the truck) after the final bleed. The exhaust down pipe is about 2” away from the slave and metal pipe, I made heat shields from thin gauge galvanized sheet metal that cover the full length of the down pipe and beyond. The slave never got beyond 160° F during the trip.

My best guess is that there must have been an air bubble in the slave or metal pipe so small that it would compress but not to affect the hydraulic action, but heated (by proximity to the down pipe) the air bubble would expand enough to inhibit hydraulic action, or something like that!
 
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