Clutch Master Cylinder Failing?

[joed]

For a while now, and only after towing a fair distance, my clutch pedal will loose resistance (pressure) if the trucks sits a while (especially overnight). If I pump the pedal a couple of times, pressure is retained and the clutch operates normally. Also, perhaps coincidentally, the pedal will periodically squeek when operated. The heat from towing seems to be a factor as it's never happened running around empty.

The fluid level is okay, although the fluid sometimes has a dark "crud" floating in it.

Is this a sign of the master going out?

Thanks in advance. Joe.

[BuffaloGuy]

I used to get this when towing extreme loads, but only when still working the motor. I realized that I must have been boiling the clutch fluid! Now with my new motor and EGT gauge and a wrapped crossover tube I've had no problems.

The crossover tube is very close to the clutch slave.

[joed]

Actually, I've already got the crossover wrapped, which does seem to help with heat. Now that I think about it, it does tend to do it while towing, but to a lesser degree. The loss in pressure seems to be greatest the first time the truck is started after sitting overnight after a day's towing?

I think I've read about a similar problem happening with the ZF equipped Duramax trucks. Not sure if they came up with a solution yet.

Joe.

[JohnC]

Could be moisture trapped in the fluid. Brake fluid is very hygroscopic (here we go again... ;) ) and if the moisture boils the clutch will get real spongy...

So, try changing the fluid: good practice to do every 3 years or so, clutch AND brakes, to eliminate the moisture. Word of caution, I recal the cylinder is some sort of composite material and easily cracked by over zealous application of torque to the bleeder screw.

[joed]

Thanks, good suggestion. I'll try replacing the fluid first and go from there.

[BuffaloGuy]

They make a DOT 5 now. It has a much higher boiling temp. Try that.

[JohnC]

Originally posted by The Buffalo Guy:
They make a DOT 5 now. It has a much higher boiling temp. Try that. Problem with DOT 5 is it won't absorb ANY water. So, whatever water vapor gets into the system puddles up and corrodes the works. Good for racing, bad for every day use, unless you're willing to change it out every few months...

[joed]

The manual calls for DOT3. Any benefit in using DOT 4 fluid?

Thanks.

[84 C2B2]

Hi,

I'm putting a NV4500 into my 84 CUCV. I'm going with the internal slave cylinder setup. Could you tell me if the hydraulic line from the clutch master cylinder to the slave cylinder can be removed. i know it can be detached from the slave. Just didn't know how it was attached to the master cylinder.

[joed]

Finally had a chance to look mine over as I haven't bled it yet...

On mine, an elbow comes off the bottom of the master cylinder and goes into a clamped, rubber house. The rubber hose then eventually juctions into a steel braided line which then goes into the bellhousing area. So it appears the line to the master can be separated.

Hope that helps. Joe.

[m-keith]

I had a similar problem on a '95 2WD gasser I used to own. It got bad enough that the clutch would not disengage. I thought it was clutch hydraulics, but it was not. Turned out that the clutch disc had worn down enough that its damper springs were hanging on the flywheel bolts thus preventing disengagement. Imagine my dissapointment when the clutch still wouldn't disengage with all new hydraulics. :eek:

I'm not sure if the manual tranny's on 4WD trucks have an external slave cylinder. The 2WD models do. If yours has an external slave, measure the pushrod travel before tearing into the hydraulics. There is a minimum spec, but I can't recall it right now. It's printed in the factory shop manual. If your pushrod travel is within spec when you experience this problem, it's time for a new clutch disc & pressure plate.

[Hubert]

JOED? So was it the master cylinder? Any other comments

I have a very similar problem: 97 K2500 5spd 112,000 miles. My clutch pedal is going soft and doesn't build resistance as early in the stroke as it use to. The clutch only has ~ 80,000 miles on it. Several components replaced under warranty by original owner.

A trans shop said usually if the slave goes bad it leaks fluid. The guy drove it and said he bets the master cylinder. I think its the master cylinder too. Because if the truck sits overnight or I have a long (40 minutes with no clutching) highway stretch I have to pump clutch up to get it to shift. After I pump it up (4-5 pumps) it does fine. It doesnt' seem to be leaking fluid and I have bled it many times now. After bleeding it several times over 2 days the fluid looks clean but whiteish snotty from little air bubbles.

One other thing is when the clutch is soft it will shudder just a bit letting out the pedal. I think thats air in the system allowing the slave to "bounce if you will". Once its "pumped up" its fine smooth and shifts easily. Just the usual nv4500 groans no other funny noises.

What esle indicates master cylinder or slave problems. Its the internal slave so I can't really measure any stroke outside. Dropping trans/tcase etc is a little intimidating with just a floor jack. Anything else I can do? Just how hard is it to get to the internal slave? Can I unbolt the trans from bellhousing or do I have to unbolt bellhousing from motor and drop the whole thing?

Any tricks to changing the master cylinder?

I too started to get the sqeaky pedal but figured it was from 300 rapid pumps over the weekend bleeding it. Its a plastic plunger mechanism under the dash is silicon spray the best lubricant for it or a grease?

Any alternative for a bigger master cylinder to make it a little easier to push the clutch?

[a5150nut]

To remove the line from the master cylinder on my 94, there is a roll pin that holds the line in the bottom of the cylinder. I have used a small piece of a nail and pliers to remove it. But be carefull, it is a plastic/composit cylinder housing. And watch for the o-ring seal.

[joed]

I ended up changing the master cylinder brake fluid which seems to have helped. I'm not sure if it's completely cured as I haven't been on any long tows since I changed it.

Joe.

[Hubert]

OK, I replaced the master cylinder this evening. The clutch pedal has more resistance to it and sooner in the stroke like it use to have. And when I was bleeding it the resistance returned much quicker pumping it back up.

The guy at the transmission shop said I would have to bleed it at installation and probably again in a day or so. I noticed the resivor bubbled small fine bubbles out of the little hole in the bottom for the piston feed when I was pumping up the system for the first time. So I guess the air in the upper part will purge itself somewhat. I think it will settle down and be fine. I'll post a follow up in a week or so if all goes well.

[Hubert]

Apparently it was my master cylinder. The new one has been doing fine for a tank of fuel and a little more. The pedal is tight all the time and disengages quickly. I can shift into reverse within a second or 2 no grinding. Shifts good. No noticable fluid loss.

I still get a little shudder / axle hop or wrap when I start off slowly in 2nd gear idling or reverse. Its more noticable when cold. Can barely feel it in 1st gear. In 2nd gear if I launch quicker with a little fuel pedal its much less to nothing. I am going to live with it until it gets worse.

Hubert