Author Topic: How to diagnose hydraulic clutch failure  (Read 917 times)

Offline JIM W

  • Newbie
  • Posts: 8
  • Newbie
How to diagnose hydraulic clutch failure
« on: February 02, 2021, 04:39:30 PM »
I have an 1985 K30 dump truck with a 9’ plow that I recently purchased for plowing my driveway.  It has not been licensed for years and was used as a site truck.  When I got it I noticed the clutch pedal does not disengage until very near the floor but it has got the job done for the last few snowstorms.  Today I was just finishing up plowing and I could tell that it was getting harder and harder to shift from low to reverse when plowing.  The pedal finally went to the floor as if it lost all fluid.  I limped home and did add some fluid but to no avail.  I don’t see any leaks and there is a good chance the clutch disc is just shot but I really don’t think it should stop so instantly.

I can pull the slave cylinder and see if the rod is actuating but, if it is not, how do I tell if it is the clutch master cylinder or the slave or?  This hydraulic clutch is all new to me.
Any thoughts or suggestions would be appreciated.  Thanks,
JIM

Offline VileZambonie

  • Global Moderator
  • Senior Member
  • *****
  • Posts: 18980
Re: How to diagnose hydraulic clutch failure
« Reply #1 on: February 02, 2021, 06:08:53 PM »
Pedal actuation would not be a bad clutch disc. Try refilling and bleeding the system first.
,                           ___ 
                         /  _ _ _\_
              ⌠¯¯¯¯¯'   [☼===☼]
              `()_);-;()_)--o--)_)

74 GMC, 75 K5, 84 GMC, 85 K20, 86 k20, 79 K10

Offline JIM W

  • Newbie
  • Posts: 8
  • Newbie
Re: How to diagnose hydraulic clutch failure
« Reply #2 on: February 03, 2021, 01:24:03 PM »
Thanks, I tried and finally noticed the slave cylinder was leaking around the push rod.  I installed a new slave cylinder and it seems to have cured the problem.  I had trouble bleeding the cylinder as it doesn't seem to pump up as a brake master cylinder would.  I had filled the cylinder before installing but it still needed to be bled.  I finally resorted to forcing the clutch arm back into the slave as hard as i could with the bleeder open and then shutting the bleeder as quick as i could.  Each time I did this the pedal push rod would extend farther than the time before.  After I drive this awhile I may have to bleed it again but, for now, I am ready to plow the next storm.

Thanks for your suggestion.

Offline MIKE S

  • Registered Users
  • *
  • Posts: 224
  • Member
Re: How to diagnose hydraulic clutch failure
« Reply #3 on: February 03, 2021, 09:07:36 PM »
Vigorously pump the pedal top to bottom about ten times. Let is set a minute or so and repeat.

Offline claude

  • Newbie
  • Posts: 40
  • Newbie
Re: How to diagnose hydraulic clutch failure
« Reply #4 on: February 10, 2021, 08:33:51 AM »
I purchased a 1985 K20 about 4 years ago. My understanding is that 1985 was the first year for hydraulic clutch. I replaced the worn out engine with a rebuilt 350 engine with headers. Since rebuilt I have replaced 2 slave cylinders with only 4K miles on engine. It is my firm belief that it is the extreme heat from my headers that is causing the slave cylinder failure. Stock exhaust heat is probably your culprit. Suggestions from this site has helped me reduce the heat failure on SC. I installed header heat wrap on lower part of driver side header. With header mounted I could only do a half assed job. Then I wrapped the lower part of header with a heat wrap designed for starters. Lastly I purchased a heat shield designed for starter solenoid & put it on then wrapped it header heat wrap. This method has worked so far. As for bleeding the SC I have found starting with engaging clutch & bleeding SC about 10 times then moving on to repeatedly pushing SC rod in forcing brake fluid and air bubbles back up into opened cap reservoir works much faster. Just my two cents worth of advice. Good luck
1985 K20, manual T, 373 gears, 208 transfer case
Rebuilt roller 355 engine, Comp XE264XR cam 212/218 @ .050, .487/.495 lift w/1.5 RR, LSA 110, Dart Iron Eagle Heads with 2.02/1.6 SS valves. 325 HP at 4000 RPM. 335 HP @ 4900.
421 ft*lbs torque at 4000RPM