Very Odd Behaviour from Clutch Hydraulics.

[George7941]

I was flushing out the fluid from the clutch system on my Sierra yesterday at work and accidentally introduced air into the system. So I did a complete bleed - pump pedal and keep pushed down/open bleeder/ close bleeder/release pedal. I got at least 95% pedal and stopped and drove home in rush hour traffic where I must have operated the clutch at least 75 times and did not notice any unusual behaviour from the clutch. After I had been home for an hour I went back to the truck and could not engage first or reverse! Pumping the pedal enabled full clutch release, so everything pointed to air in the system. I bled the system once more and got 100% pedal and the clutch was fine today when I drove the truck.

So, how did this one hour break introduce air into the hydraulics?

 

[MDettling]

Tons of little bubbles dispersed, and you had an air bubble. Re-bleed?

 

[ls1mike]

You didn't have it all out.

 

[ragtoplvr]

Put a block in there to hold the clutch petal down overnight.

The pressure makes the bubbles smaller, and they can work up and out.

Rod

 

[George7941]

"You didn't have it all out" is the obvious answer but it does not explain why I had a normal pedal on the drive home. If the air was there, would it not have affected shifting into first on the drive home like it did after the hour long shut down? There was a distinct deterioration in the clutch release during that hour, it wasn't something just in my imagination.

The only explanation I have is that a thousand tiny bubbles don't compress as much as the one large bubble they aggregate into, given time and rest. I don't know how if there is any truth to it.

About pressure making the bubbles smaller and them working out, I have never heard of it but am willing to try that next time. I have rebled the clutch and have a normal pedal now. I hope it doesn't affect the seals in the m/c or the slave since they don't normally see high pressures for hours at a time. The slave isn't easy to change since it is concentric and the tranny has to come out.