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Slipping clutch


BOZO

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My 1974  620 has a fairly new clutch and pressure plate in it, but recently the clutch started slipping. I noticed the clutch was engaging almost at the upper end of its pedal travel so I adjusted it under the dash to get more free play. The truck ran properly for a few days but now its slipping again and the clutch is again engaging at the end of its travel. I'm thinking (hoping) something is wrong with my clutch master cylinder. Like its not returning all the fluid. My slave cylinder is new also. Has anybody else had this problem and how did you fix it? This problem is about to put me afoot as my backup ride is broke too. Any help would be MUCH appreciated.

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If you suspect that pressure is trapped in the hard line between master and slave, loosen the line fitting at the master end. If fluid squirts out and the slave pushrod moves forward, then yes.

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  • 3 weeks later...

Back again. I installed a new clutch master cylinder and the problem went away until this afternoon. Coming home from work, it started again. I pulled over and loosened the line at the master cylinder and fluid squirted out again. After that it worked properly. Could I not have the pedal stop adjusted right? My Chilton manual is not very vague about adjustment. I have it adjusted so the cylinder comes all the way back and then some. I know this is not in the clutch itself because when its working right it will spin the tires and will not slip in any gear. I can tell the engagement area moves up in the pedal travel over time just as it did before.

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That should still work.

 

Trapped pressure indicates that the piston inside the master is not returning fully and allowing the pressurized fluid ahead of it to return to the reservoir. Pedal adjustment seems ok and it appears that there is no reason for this not to work. Unless someone has another idea I can't take this any further than to say the master is at fault.

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I do  not know about 620 trucks, but 521 trucks had a return spring on the slave cylinder.  They also had an adjustable threaded rod that pushed the throwout bearing fork.  You set the free play at the slave cylinder. 

When slave cylinder free play was set, the master cylinder push rod adjustment was then set, and after that, you only had to adjust the free play at the slave cylinder.

 

You should see be able to see the fluid return back into the slave cylinder reservoir, when the clutch pedal is fully up.

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521ClutchSlave.jpg

 

 

The early ones with L16 and the 521/510 type short 4 speed had the hole in the clutch arm, adjustable push rod and the return spring. The '74s had the first year of the long 71B trans without the spring.

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Thank you all for the prompt response. I will do some readjusting on the pedal free travel and see what happens. I know there is no adjustment underneath the truck so I will see what I can do under the dash.

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If all else fails I may try adding a spring down beside the slave cylinder as the earlier trucks have to help return the fluid. This is the first vehicle I have owned that has a hydraulic clutch, so this is new territory for me.

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Hell if you are going to do that just replace the master. You can do that in 15 minutes and 10 min to bleed it. How much will a mechanic charge for his time? $80 an hour? Half an hour would pay for the part.

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  • 2 weeks later...

In the past my clutch would go to the floor.  Sometimes I could pump it up. This was after replacing both master and slave from autozone. I finally got a good master and slave and havent had the problem.

 

I suspected about 50% chance they sell you a bad one.

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  • 2 years later...

oh my god im having the same problem i have a 1973 620 i replaced my 4 speed with a transmission from a 1983 280zx i put in a  73 original clutch kid but kept the old slave cylinder on

i didn't adjust anything or had to bleed the cylinder it was running good for about 4 days and then it started

 

i couldn't shift to first gear or any gear at all unless i turn my truck off shift to 1st and start the truck with my foot pushing the clutch in

 

then it got worse now i had to turn the truck off shift to 1st gear but when i would turn my truck on the truck would just take off running

even thought i had my foot pushed all the way down

 

i adjusted the rod thats inside the truck on the clutch pedal bringing the rod almost all the way out it worked for  a day 

 

but i notice the it would slip a little bit on 3rd and 4th gear just a bit was running nice tho

 

the next morning it went from running nice 40 minutes later started slipping on 3rd n 4th gear n within an hour the tuck wasn't going more then 15 per hour on any gear

 

i changed the slave cylinder readjusted the rod on the pedal but is still slips on 1st 2nd 3rd n 4th i don't know what else to do but is really getting the best of me i need help asap please answer back with any suggestions thanks 

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oh my god im having the same problem i have a 1973 620 i replaced my 4 speed with a transmission from a 1983 280zx i put in a  73 original clutch kid but kept the old slave cylinder on

i didn't adjust anything or had to bleed the cylinder it was running good for about 4 days and then it started

 

i couldn't shift to first gear or any gear at all unless i turn my truck off shift to 1st and start the truck with my foot pushing the clutch in

 

then it got worse now i had to turn the truck off shift to 1st gear but when i would turn my truck on the truck would just take off running

even thought i had my foot pushed all the way down

 

i adjusted the rod thats inside the truck on the clutch pedal bringing the rod almost all the way out it worked for  a day 

 

but i notice the it would slip a little bit on 3rd and 4th gear just a bit was running nice tho

 

the next morning it went from running nice 40 minutes later started slipping on 3rd n 4th gear n within an hour the tuck wasn't going more then 15 per hour on any gear

 

i changed the slave cylinder readjusted the rod on the pedal but is still slips on 1st 2nd 3rd n 4th i don't know what else to do but is really getting the best of me i need help asap please answer back with any suggestions thanks 

 

The zx transmission had a release collar and bearing on it? did you use it, or the one on the old four speed????

 

If you used the collar and bearing that was on the four speed, then the slave cylinder needs the air bled out of it and all your adjustments returned to where you started.

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datzenmike thank you for th quick respond and the answer to ur question is no i didn't use the collar from the 4 speed unfortunately my truck was at my moms garage for

 

about 10 years withe the 4 speed seating around on the floor till it finally made its way to the dumpster with collar and bearing so i used the collar that came with the zx n the original bearing that came with my clutch kit  :confused:  :confused:  :confused: 

 

       

 

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This is the problem. The collar for the zx transmission (usually a 225mm diameter clutch) does not match the clutch on your 620 (200mm). You need the collar for the 620 clutch assembly that you have.

 

Clutches come in many sizes and heights. The slave is limited to how far it can move.(about 1.18") To allow for all these differences the collar length is matched for the intended clutch use. Shorter pressure plate height needs longer release collar. Taller needs shorter.

 

 

The Nissan part number for the clutch sleeve for the '73 620 is 30501-N1601

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since you got the correct bearing on there (I assume its a 200mm kit that same as a 521 L 16 kit part number)

 

You can feel if the slave is tight when the clutch fork is pushing against the rod.  I would make a shorter rod for the slave and see if this makes the clutch not slip.

 

if clutch fork arm wiggles then it wont work and assume the clutch is out already and its released. But if tight like in above statement one can maybe shorten the rod as I assume its putting pressure on the clutch fingers

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i tried putting the spring in there to see if it would help but all it did was make the clutch pedal a little harder to push but the slave cylinder looks like is all pushed out

and thats with 1st gear in no one pushing the clutch n the truck off

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