Clutch fluid question

Yep, hydraulic with common reservoir. Most recently (and most relevantly) would be my 2013 Mazda3 that got wrapped around a Nissan last month that has spent 3 winters in the same driveway my truck now relaxes in, and my wife's 2015 Impreza that sits next to it. 3 Massachusetts winters seeing the same temperatures without even a stiffer pedal throw much less lagging return, yet I've already had 3 or 4 days in the last month where the truck needs a few miles to fully wake up the clutch.

I totally get that this is a truck and those are compact cars so the comparison isn't apples to apples, but until I got this truck I had never even heard about this sort of problem being a thing outside of maybe stories of the deep Canukistani north.

 

Well, I'm at Canukistan, not a deep North, but at -37c time to time... I consider stiff hydraulics at this temperatures more-less normal, just need more often inspections for possible leaks and more gentle operation in the beginning at this temperatures...
But, good luck to you with research about viscosity and different fluids, please keep us all posted!

 

After 3 pages of discussion, I may abandon the idea completely. The main reason is that the many things I've learned about these trucks since I bought it has given me the distinct impression that Toyota spec'ed the seals for at least one component in the hydraulic system to only just meet the maximum clearance requirements to hold in DOT3 fluid. Because the bastards are cheap like that. I have a feeling that if I drop in DOT4 LV or DOT5.1 after a arbitrary period of time I'm going to start to see weeping on the master or slave cylinder, or brake booster, or brake calipers, or ABS relief valves or something...

 

If you're going to try brake fluid, Amsoil Dot 3 is 817 cSt @-40C. I am not sure how that is compared to other products. I used ATE on the track in Florida with good results but we ain't in Tampy town and we ain't drivin' IS300s and 86s no more.

I experienced this phenomenon today in east TN after a day of below freezing temps caused the clutch to be a little slow coming forward. I think the low was 16 and the high was 20 something. After a few minutes, it became less and less slow. Freezers do interesting things to vehicles.

 

Yep, that's exactly the phenomenon I'm dealing with. I'll have to look at the Amsoil, and maybe a few others to see how their specs compare. I'm also going to have to find out what brand goes into them at the factory.

 

You wont have problems, the kinematic viscosity of all the fluids DOT3, 4 and 5.1 are the same at 100 degrees and they are 100% compatible. Think of it as engine oil. 0-30,5-30,10-30 and 15-30 or even straight 30 oil are all a 30 weight oil. The difference is in the first number. 0,5,10 or 15 which is the viscosity of the oil at cold temps This is the same for kinematic viscosity. DOT 3 and 4 are around 1500 at -40c where as 5.1 and even super 4 is about 750-900 at -40c , but they are all 1.5 at 100 degrees.

5.1 is more hydrophobic and will need to be flushed more often but it wont effect the seals and cause weeping of the master cylinder. it also has a higher boiling point.

If you live in a cold climate you run a 0 weight oil no questions asked. Why? Because at cold temperatures its thinner and it flows better. Brake fluid is the same. It is just a hydraulic fluid. The thinner it is the better it flows. That's why the clutch gets better after driving in the cold. The fluid heats up and flows better.

 

Hmm, I have a hard time arguing with that logic. I'm not sure why I didn't think to look at the 100C viscosity and realize that is the minimum viscosity the seals should be able to retain. I'm not 100% convinced though, since the follow-up question in the back of my mind is "what effect does the temperature range have on the seals themselves?" Still, you make an excellent point, and I thank you.

 

I think it was only single digit lows in our part of the midwest over the weekend. I would say the clutch staying on the floor 1-2 seconds after my foot leaves is most disappointing. All our old ford hydraulic clutches are always slow in the cold but not near this bad. It makes sense to me that if DOT 4LV is rated FMVSS no. 116 DOT 3 compatible it would be okay to use, and after finding this discussion I am more likely to try it.