Brake fluid

Brett · Post by **Brett** » Fri May 23, 2008 4:10 pm

Well, I bought some new brake parts. New master and wheel cylinders, and new shoes. Just wanted to know if its ok to use regular DOT3 or 4 brake fluid, or if I need to use the original mil spec stuff. My gut is that that is ok, and that the silicone fluid would be harder on seals. BTW they are USA made Parts from AB Linn.

Thanks
Brett

Nickathome · Post by **Nickathome** » Fri May 23, 2008 4:40 pm

I've been using Prestone Dot 3 synthetic brake fluid and so far have no complaints.

Lifer · Post by **Lifer** » Fri May 23, 2008 6:24 pm

Silicone fluid is okay but...!

(1) All traces of non-silicone fluid have to be flushed out of the system. If the two fluids mix, you get a gooey glob instead of a fluid.

(2) Silicone will dissolve any natural rubber components very quickly. If all your seals ,piston cups, and flexible lines are all synthetic[i.e. neoprene], it works just fine. If you have even one natural rubber component, you will encounter a problem sooner or later.

The earlier models were designed and built before silicone fluid came along, so I think I'll stick with the original stuff myself when I get that far along.

g741 · Post by **g741** » Fri May 30, 2008 5:05 am

Hello: My two cents worth regarding Silicone brake fluid.

I had trouble for years, literally, with my brake cylinders becoming corroded and frozen up internally while using plain old brake fluid in my M37. Cleaning it all up and installing silicone fluid solved the corrosion problem.

I put silicone brake fluid in my 1941 Chevrolet 1.5 ton in 1991 before going up the Alcan Highway in 1992. In 2004, prior to shipping the truck to Europe to take part in the 60th of D-Day, I checked the wheel cylinders. All were perfectly OK, no problems. I did not check the master cylinder.

Now, here is the only problem I have seen in the last 10 or 15 years regarding the use of silicone fluid in the Chevrolet and in the M37. The piston cup that the piston pushes on has a brass washer molded into the rubber cup. At least the newer ones do. Early on there was a separate washer. Now, I have had two failures of this cup only in the M37 and the same cup failed on the Chevrolet while in Europe. I had a spare MC in both cases so I was not stuck on the road in either case.

For some reason, the rubber compound they use, or the act of molding the washer into it causes it to fail, even when using silicone fluid, but only after 8 to 10 years. I will begin changing out that cup on a maybe 5 to 7 year cycle. All the other cups seem to hold up indefinitely. Sid

rixm37 · Post by **rixm37** » Sat Jun 07, 2008 9:07 am

Sid that sounds incredible that the silicone fluid lasts that long. I am just tearing out my whole brake sys to do Ray S. disk conversion and new rear wheel cyls.and master cyl. My question is how do you flush out the brake lines on the M37 to prepare to swtch over to silicone?

Lifer · Post by **Lifer** » Sat Jun 07, 2008 9:40 am

The way I do it is to remove all wheel cylinders and rebuild them (which gets rid of any fluid in them). While they're off, disconnect the brake line(s) at the MC and blow them out with compressed air. If installing a new MC, hook up the brake line(s) to it, fill with silicone fluid and pump it through the lines to the wheels, topping off the MC as necessary. Then, blow the lines out again and repeat the flushing process. If using the old MC, rebuild it and do the flushing process described above. Then, install your new/rebuilt wheel cylinders, hook up the lines (replacing any rubber lines), refill the MC and bleed the brakes as usual. If you do all that, there should be none of the old DOT3 fluid in the system.

k8icu · Post by **k8icu** » Sat Jun 07, 2008 7:16 pm

silicone and dot3 don't play well together, but they won't gell up like the old motorpool sgts tail says they will. What happens is the dot 3 floats on top of the silicone and then can allow water get into the system and start the corrosion problem.

I have had silicone in my M151 for the past 23 years. Not one problem to date. I have offroaded the vehicle, gotten stuck in sand up to the diffs, gone through WV car washes and not one problem!

If you are replacing all new parts you except the steel lines then what you can do to clean them out is do like discribed above with the air, but then push rubbing alcohol through the lines then hit them with air again and you'll be good to go. When the army made the switch to the silicone the instructions were to oppen all the bleaders and push the old stuff out, then run the RA through till dry and then put the silicone in.

But there is nothing wrong with going with what was original to the truck either.

rixm37 · Post by **rixm37** » Sat Jun 07, 2008 9:32 pm

Well thanks guys . I won't be needing to flush out the lines because after pulling it all apart I found mounds of rust everywhere. I stuck a q-tip up the hard line and it came back full of rust particles and brown goooo. The wheel cyls were trash. It was so bad I took pics which I will try to post later.
So come Monday gonna order up a whole set of new lines. But hey is it easier to just bend up your own?
Regardless,everything will be new.
SOO where do you find silicone flluid? Is it at the big chain auto parts stores?

Lifer · Post by **Lifer** » Sun Jun 08, 2008 4:21 am

Bending your own lines isn't too difficult, but tight bends can be a problem. The tubing will likely kink if you don't have a bending tool. You'll also need a double flaring tool. (single flares are ok for fuel lines, but brake lines are under a lot more pressure, so you need the extra strength at the coupling.)

Silicone fluid is available at all auto parts stores. Farm King, Tractor Supply Company, and other such places will also have it in stock.

k8icu · Post by **k8icu** » Sun Jun 08, 2008 6:52 am

Yep what Lifer said....Yes bending your own brake lines isn't difficualt, but sometimes it is just nicer and easier to have the right line already made to just pop on in.

rixm37 · Post by **rixm37** » Sun Jun 08, 2008 1:37 pm

Ya that makes allot of sence