Trans shifting issue

[Sheener]

4 speed manual in my 86 3/4 ton 4wd with 460. When cold everything works fine. After driving and it warms up it gets harder and harder to shift and some grinding during change. Is this a slave cylinder issue and if so, are you supposed to change the entire system? Master, tube and slave cylinder? I did check transmission fluid level and it is fine

[Gary Lewis]

It sounds like the clutch isn't fully releasing when hot.  And that could be due to a clutch linkage problem, including the master and slave.

Have you checked that the clutch master has enough fluid in it?  There's a dust boot under the lid and you have to pull it out in order to see the level.  And you shouldn't fill it up too much or it'll run out when you put the boot in.

[Sheener]

Fluid level is actually too high, it comes out when I take the cap off.

[Gary Lewis]

Have you taken the boot out?  Not saying you are like , but when I got Big Blue it shifted like yours.  When I pulled the cap on the clutch master it looked like it had plenty of fluid, but that was just fluid someone had added on top of the boot.  After pulling the boot and adding fluid to the reservoir itself it started shifting much better.

[Sheener]

Pulled the boot and the fluid level was above the step on the master. Had the boss of me depress the pedal and measured movement at 7/16”.

[Gary Lewis]

I don't know what the movement should be, so can't comment.  But at least you've found that you have fluid.  

Lets let someone else chime in as I'm not sure where to go next.

[grumpin]

In my old 88 F250 with the ZF tranny, someone overfilled it and it did not shift good at all.

What type of fluid is in the tranny? Does it call for ATF?

[Sheener]

80 weight. Clutch acts like it is not disengaging. I have to turn the engine off and put it in gear. With clutch depressed this afternoon and in neutral, I could push the shifter toward granny low and the truck would creep forward and stop when I released pressure on the shifter.

[grumpin]

Yes, seems to be engaged.

[Littlebeefy]

The firewall cracks badly where the master cylinder bolts on. Have you verified that the firewall isn't broken and flexing with the clutch pedal?

[Sheener]

Firewall seems solid. Has anyone ever experienced the hydraulic line getting soft and weak when underhood temps go up? Being plastic, i wonder? Will check that next time I am under the truck.

[Gary Lewis]

Chad is right about the firewall, and it is hard to see it move.  The best thing to do is to use something to measure any movement.  Some have used a 2x4 on the radiator support back to the firewall to see if it moves.  Others have put a tape on the radiator support back to the firewall.  Then push the clutch in to see how much the firewall moves.

[Sheener]

I did check that yesterday, just rocking the master cylinder as I did the first time did not show much movement. Had the boss of me push the pedal while I measured. Between 1/8-3/16”. Ordered a reinforcement plate from bronco graveyard. I will add that before I change any parts. Thanks for the help.

[Littlebeefy]

Good luck with the reinforcement plate. I got it in but I abandoned the part that has to be installed under the cowl (still works fine and my firewall was definitely cracked). If I recall correctly, you will need to pull the entire pedal assembly; that would give you a good opportunity to check and replace those bushings.

I had a 4sp (T170) in my 84 before I swapped it out. I never had any problem with my hydraulics for it, but it's possible it had replacement parts from a PO.

This is a shot in the dark, but is it possible you are using the wrong type of brake fluid for that system and it is failing at temp?

[Sheener]

That would be worth changing and not a big job. Thanks.

[Gary Lewis]

I should have pointed you to TSB 90-16-07 in the first place.  It takes you through an inspection process for the clutch linkage and then discusses installing the reinforcement plate.

As for replacing the fluid, it is a pain to bleed these systems, so if I wanted to replace what is in there I'd pull it down to just enough in the bottom of the master to ensure air does not get in and refill with new.  then I'd pump that through and refill as it gets low again.  After a couple of rounds of that you'll have new fluid in and no air.

[Sheener]

Thank you Gary.