'78 Clutch Engagement Issues

[Bojanglez280]

For any of you who have kept up with my progress, you probably already know I have gotten the car running. Today I got my gas tank back from Crawford Z Car Service in Nashville. I am near the point of having a "road-test" worthy ride. Here is where I am at....I have replaced the clutch master cylinder, the hard line that runs from it to the slave, the rubber hose at the slave, and the slave cylinder. I have bled the system and gotten clutch pedal pressure (there is tension in the pedal; compared to the "light as paper" feel it had beforehand). While the car is running (I have it on jack stands), I push in the clutch to the floor and attempt to put the car in gear....the rear wheels begin spinning before the car goes into gear. It will not go into any gear....it feels like the clutch never disengages through the swing of the pedal. The pedal is 8 3/4 in from the floor (measuring from the firewall to the top of the pedal cover), but there feels to be far too much free play at the top of the swing...maybe 2 inches or so compared to the 1/4 inch recommended. So my question I guess is 2 fold - 1) How do you shorten the free play? 2) What else should I be trying to do to right my wrongs? Reminder, I am a school teacher by day and a pick and learn as I go project car worker by nights and weekends. Just share that with you to help "dumb down" any advice you may have to offer. Thanks for your help in advance. As always, I am very grateful.

Jamie

[Blue]

Hi Jamie,

Here is a quick overview of hydraulic systems:

1. A system is made of two plungers at each end and a line connecting them. Inside is a fluid that is contained by seals. It is a closed system.
2. A push on one plunger causes a corresponding movement of the same distance and force at the other end's plunger. (as long as both plungers have the same surface area (piston diameter),
3. By making the piston of the pushing plunger bigger, more fluid will move and the far end plunger will move forward more. (note 1)
4. By making the piston of the pushing plunger smaller, less fluid will move and the far end plunger will move forward less. (note 2)

Note 1: For a car brake system, a bigger diameter pusher will cause the brake pedal to feel harder faster as more force (pressure in the system) is "caught" by the piston face as it is proportional to the surface area (i.e. psi and bigger has more i)


Note 2: For a car brake system, a smaller diameter pusher will cause the brake pedal to move farther but not feel as hard to exert the same braking force on the wheels... again it is due to the psi geometry and the ratios of the surface pushing and the surface being pushed.

The clever way pressure systems works to amplify applied pressure:
If you have a pressurized chamber (like a diver's air tank) and two plungers attached (one which has a surface of one square inch and one that had a surface or two square inches), then if you fill the take with air to 1,000psi, it will take 1000 pounds of applied force on the small plunger to hold it in place. It will take 2000 pounds of applied force on the larger plunger to hold it in place as the surface area is 2 sq" and the pressure is 1000psi so the force is 2000 pounds.


In your case, you must ensure that the master clutch plunger moves sufficiently far by the pedal so that the slave plunger moves sufficiently far enough to move the clutch yoke. Sometimes the problem is that the master piston is adjusted incorrectly and its plunger rod's resting state is actually well into its excursion. If this is the case, back the rod out towards the cab as far as it will go.

It could also be that your replacement master is made to a high Mainland China quality and the rod is too short or does not have enough thread to allow for full adjustment. If so, you may have to scavenge the old one or get a longer one from another vehicle (fyi I had this problem with a RAV4 master and used a rod from a Z to make it work).

[Bojanglez280]

Update,

We have adjusted the master piston as far as possible to make for as much piston travel as possible from pedal pressing....We have bled the system probably 30-35 pumps. We have not seen an air bubble in 15-20 pumps. The clutch pedal is stiff, as in is does not feel spongy. Yet, upon full pedal to floor extension, the slave cylinder push rod is only moving approximately 3/4 inch. Based off of this information, is the problem as I expect that the O'Reily Auto Parts master cylinder replacement one "likely made in China" has too short of a push rod? If so, you mentioned a Rav4 master.....what year's Rav4 push rod would work? Or what other year/model/make could I try?

Or, what else could I do and or look at with my current setup to try and make things work? I feel like it is so close, yet so far away. Thanks in advance for your assistance.

[Bojanglez280]

Next update....on another thread I found a mention of a guy getting a M8 bolt extension and a coupler to extend the pushrod. We did that...it worked...the slave movement went from 5/8 inch to now 7/8 inch movement. FSM recommends 1 1/3 inches of movement....How do you suppose I squeeze out the remaining necessary distance?

[Bojanglez280]

Is there a master cylinder with a longer piston that I need? My master cylinder is a 5/8 bore from O'Reilys....if I need a new one that is fine. If it has a longer piston, that seems to be the solution.

[Zed Head]

Have you checked the clevis assembly for play and wear? If I was installing a clutch MC, knowing what I know now, I would eyeball the relationship between the clevis pin holes and the pin hole on the pedal before I inserted the pin. The MC has an internal spring that pushes the piston out to where it should be in the cylinder. If it's pushed out too far, you'll run out of travel in the cylinder, if it starts out too short, you'll run out of travel on the pedal. The starting point should be to have the clevis pin holes lined up before inserting the pin, then fine tune it. If the pedal is at the top of its travel and the MC piston is at the start of its stroke, that's the maximum travel you'll be able to get. Easy to verify by removing the pin and seeing where things are. Might have to use a mirror.

There's probably a stop screw under the pedal also that can be adjusted. One more thing to check.

[Bojanglez280]

To feel like we got something accomplished today....we mounted the gas tank back to the car and bled all brakes after installing new bridge tubes on the front. The rear passenger wheel cylinder blew while pumping the brakes, so I have ordered new wheel cylinders from Rock Auto. I am absolutely baffled by the clutch situation. I am starting to lean towards the one option I didn't want it to be, that the pushrod on the new Master is not as long as the original. I kept the original master cylinder for approximately 6 months, until last week when we had an appraisal on our house and I threw it away to help "clean up" the garage. You know those moments where you wish it were possible to give yourself a swift kick in the anus?? This would be one of those moments. That is at least, to the extent of my knowledge, what I think the problem is. Where can you go about getting an original pushrod? Unless anyone has better ideas, that is the next thing I am going to try. Zed Head....we checked the clevis assembly, and everything seems fine. I have not looked into the internal spring. I guess you could have something there. That is something I need to look into soon.

[mgood]

Look to post #2 and reread #3 and #4 from Blue this might be a solution go to a bigger diameter master.
Just a thought.