A few thousand miles ago, I replaced the slave cylinder, master cylinder as well as the hydraulic line that connected them. I didn't change anything else.

Everything was going fine until about two days ago when the temperature dropped to 15 °F. Then it got harder to shift. The next day It wouldn't go into gear while the engine was on. Checked in the bell housing and see that it had fluid in it.

Is there something other than the slave cylinder itself doing it? Like not getting the hydraulic hose on perfectly. Also if it is the Slave then what all should I change while I am in there? I don't really want to have to do this a third time, so I am wanting to change what I can change and hopefully not have to do it again for the life of the truck. Is there a recommended brand or type of Slave that I should get? The one in it now is a Duralast from AutoZone.

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1992 Ford Raner with a V6 3.0 and a 5 speed manual transmission.

There is your problem get a slave from Ford

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1999 Ranger 4.0 and extreme Snap-on tool fetish

I believe there is a guy on youtube that has a video replacing an Autozone slave that was less than a year old. I just put a Perfection Clutch slave in mine. It is nicer than the factory unit as it made from billet aluminum and not plastic like most of the others, including the OEM.

Could it possibly be trans fluid leaking out a front seal?.

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2000 Ranger Extd Cab XLT, 4X4 Off Road, 4.0L, Auto, 8.8 - 4.10's, 01 grille & lights swap.
69 Mustang, modified 5.0, & 5spd.(project)
99 Explorer Sport, 4.0 SOHC, Auto, 8.8 - 4.10's, PW, PL, PM, Cruise, Tilt. (Wife's DD)
05 F-150 2WD Regular Cab. ( My DD)