'91 clutch issues

[TNewton]

Hey guys, been a long friggin time since I've been on the site, but I thought I'd offer some brain food. I've got a '91 X Sport, 2" susp. lift, fresh body, 31's, daily driver, nothing crazy. Truck has 300K on the clock (no misprint, 300K.... really). Just rebuilt the trans (M5R1 5 spd) abt 15K ago. Trans is doing great, but in the last 2 wks, clutch free play has increased to abt 4-5", and clutch engages approx 1" off the floor. It sux. Ambient temp in that time has been below 30 deg F. Of course I replaced the slave cyl- T/O bearing when I did the trans, but now I'm wondering if it's taking a dump on me. Clutch is a Hayes Super Truck series, and the slave came from a reputable local parts house (not AutoZone or Advance), for whatever that's worth. I'm hoping it's just a bad master cyl, but I see no external leaks or other sign of failure. It's obvious it has air in it from somewhere, but air in usually means fluid out, and I'm not seeing it. Everyone I've talked to says they've never seen a master go bad, but they've never seen one with 300K, either. Fluid is still full, but is black, and I did flush it when I did the trans. I'm planning on throwing a master and line at it, but I'm just wondering if anyone else has had a similar issue. If I gotta pull the trans again just for a bad slave, I'm gonna pull my friggin' hair out. Thanks for any insight you've got.

 

[manaen]

on these master cylinders the only way to get all the air out is to remove the cyindel from the firewall and invert it while pumiping it. The angle it mounts to the firwall makes it impossible to get all the air out. It's not super difficult, just time comsuming. I would start there by bleeding out the entire system to remove any air. Sometimes if your master cylinder seal is wearing out air can be sucked in around the seal on fast pedal movements. The cold can harden the seals and make this problem worse. It may be time for a new master cylinder..

when bleeding the cylinder go slowly to ensure you don't suck more air in the lines.

 

[Anime]

If it's the slave cyl, usually you'll have trouble with the clutch not engaging.

Since it's more of an issue related to the pedal, I'd say your guess is correct and a new master (and maybe hydraulic line) is in order. Just be sure to get the master and line for a 91-92 and not one for a Ranger or 93-97 Explorer. I'd suggest a Ford/Motorcraft one from the dealer or FPN, but since you went aftermarket on the slave you might just go that way on the master as well.

I'd sell you my master cyl, I've got a unit for a 91-92 in mine that I'll be swapping out for a 93-94 master due to fitment and clutch travel issues as well, but it might be a while.

 

[TNewton]

Thanks for the input. What's the difference between the master on a '91-'92 X and the later ones? Different displacement or different mount?

 

[Anime]

The mount is the same since they bolt up in place of one another, I'm not sure if the displacement is any different, but they seem to be the same size housing so I'd assume not.

The main difference is the hose routing, the hard line on the 91-92 master really wedges into the 93 from the firewall and down to the transmission, where the correct master fits perfectly. There's a notch in the firewall seam for the 93-94 master hose, and I couldn't get the 91-92 line oriented to use the notch, so it really looks bunged on there and is obviously not the best fit. The 93-94 might fit better on the firewall of a 91-92, but might also have the same problem if there's a notch on the seam it can't be positioned into.

 

[TNewton]

Just a quick update- my clutch issue suddenly fixed itself yesterday.... freeplay went back to normal, engagement was back to normal....sweet! Until I was sitting still at an intersection and almost hit the van in front of me cuz the clutch magically decided to start engaging itself.... dammit. Now I have basically no pedal at all and I gotta drive it like a Freightliner. Gonna put her down this weekend and figure it out once and for all. Back to driving the ol' Dodge for a bit.... gotta love 10 mpg..... I'll let y'all know what I find out.