Synthetic or silicone brake fluid?

[adkrover]

I am replacing a leaking clutch master cylinder and was wondering about proper brake fluid. The previous owner used synthetic fluid that is blue in color. At NAPA they are trying to sell me on silicone fluid because they claim it won't absorb moisture. Does anyone know if synthetic or silicone is compatable with the seals in the clutch and brake system? Any good reason to go with either?

Thanks

[yorker]

If it is blue or purple it probably was silicone brake fluid. I've used it for 10+ years with no problems.

[Tim Smith]

I used silicone on a IIA that had a non syncro box. I wouldn't advise it!

In my case the fluid was definitely "slower" and it made double declutching a very slow and in effect, useless process. If your system uses two reservoirs, then I'd consider using the silicone in the brakes and leave the clutch as the standard dot 3 or 4.

Silicone isn't cheap but it doesn't mix with water, so that is a good thing. It's a little thicker (mentioned above) so it tends to not leak through the seals as much. It also has a high heat tolerance, so it's good for racing but for our trucks that doesn't really apply.

Thats my .02

[LaneRover]

I used silicone on a IIA that had a non syncro box. I wouldn't advise it!

Silicone brake fluid in a gearbox? Wow I thought that would have speeded up the shifting AND your gearbox's general usefullness!

[Jeff Aronson]

I think what he meant was that on a Series II-A with the non-synchro 1-2 box, the clutch needs all the help it can get. The use of silicone brake fluid in the clutch hydraulics seemed to be his problem. Silicone fluid is not compatible with other brake fluids and with some older rubber seals. Maybe the clutch lines, master and/or slave cylinders were not fully drained when the silicone fluid was added.

Actually, I'm told that Amsol's synthetic grear old really does help with shifting in cold weather, and lasts longer in the box/transfer case, too. If you stayed in Maine instead of southern California in the winter , you would remember the effort required to move the gearbox first thing on a winter's morning.

Sigh..yes, I'm jealous. It's snowing here in Maine today.

Jeff

[yorker]

I don't know why you had trouble with it in your clutch system? I used it in both in mine and never had any issues double clutching- I know a lot of other people who have used it without issues either. I usually buy it by the gallon from Military surplus dealers- it is usually $30 a gallon which isn't bad IMHO.

[Tim Smith]

Jeff is right. I was using it in the hydraulic system, not the transmission. Maybe I should clear up my post.

In my case, the clutch peddle was way slowed down once we switched to silicone. You could take your foot off the peddle and would have to wait for it to come back up. I'd say, that it would take about 3-5 seconds for the peddle to come back up on it's own accord.

Perhaps we didn't get all of the old fluid out but I thought that mixing the two wasn't a bad thing. Can't remember though as that was a long time ago...

[LaneRover]

Silicone brake fluid in a gearbox? Wow I thought that would have speeded up the shifting AND your gearbox's general usefullness!

Just having a bit of fun

[Daurie]

I remember reading on here at one point that using "standard" DOT spec fluids in the clutch and brake system causing the slave cylinder seals in both systems to deteriorate.. I don't know if this applies to cylinders rebuilt with newer rubber but may be worth researching. Seems like it was in a post about an individual who was battling incessant brake cylinder leakage issues. I bit the bullet and bought the "recommended" lucas stuff from our host just to be safe.. I'm beginning to see that it depends on YOUR truck more than the fluid used. I've noticed people using the same specification fluids with different results!!

Good luck!

[Daurie]

.

Actually, I'm told that Amsol's synthetic grear old really does help with shifting in cold weather, and lasts longer in the box/transfer case, too. If you stayed in Maine instead of southern California in the winter , you would remember the effort required to move the gearbox first thing on a winter's morning.

Jeff

Anything Amsoil is good IMHO!
And I don't think I will soon forget the effort required to remove my gearbox... even in my warm shop!

[yorker]

I remember reading on here at one point that using "standard" DOT spec fluids in the clutch and brake system causing the slave cylinder seals in both systems to deteriorate.. I don't know if this applies to cylinders rebuilt with newer rubber but may be worth researching. Seems like it was in a post about an individual who was battling incessant brake cylinder leakage issues. I bit the bullet and bought the "recommended" lucas stuff from our host just to be safe.. I'm beginning to see that it depends on YOUR truck more than the fluid used. I've noticed people using the same specification fluids with different results!!

Good luck!

Castrol LMA is the old standby because so many people have had issues with other brands of DOT-3 eating the rubber seals in British brake systems.