Synthetic Brake Fluid

The synthetic fluid does not absord moisture. You will not need to change the fluid to avoid the corrosion problem.

Synthetic is more expensive to purchase.

Don't do it.

You run the risk of incompatibility with the rubber components of your brake system. The silicone based DOT 5 fluids are not always kind to the brake system. The seals in the master cylinder and/or wheel cylinders may swell and seize up or deteriorate and leak. It's much easier (and cheaper) to simply fully bleed the brake system every two years or so. It seems most people NEVER change/flush their brake fluid with surprisingly few harmful effects for the first 8 to 10 years. I've looked at numerous master cylinder brake fluid reservoirs where the brake fluid had the appearance of a fine stout.

If it wasn't designed for DOT 5, don't use it.

Cheers !!

The first thing is, you didn't say how long the truck had been sitting or how long since the brakes had been looked at or the fluid changed. If the fluid is as antique as the truck, you should expect to have to replace the entire braking system just as you should be replacing all the rubber fuel lines etc.

DOT 5 may not absorb moisture, but moisture is still there to do the corrosion. The whole idea behind absorbing moisture is to change the fluid regularly. With either fluid, you are going to be changing your brake fluid regularly. Most manufacturers now recommend every two years.

DOT 5 has a higher boiling point. In theory you could have your brakes on fire and the fluid would still work. Most DOT 5 users I know of use it in the race car and not the street car.

DOT 5 seems easier to contaminate and is more succeptible to ruining rubber parts on older brake systems or rebuilds. You need to make sure that the master and slave cylinder components are DOT 5 compatible.

DOT 5 is Expensive and not always that easy to find.

In short I can't think of one good reason to use DOT 5 over DOT 4 or DOT 3. You would have to change fluid anyways so use the regular fluid. You save money and avoid problems with regular maintenance which you were going to do anyways because when done, the truck will be your pride and joy.

My two cents for what is worth.

All the Harley-Davidson dealers have DOT 5, it's the ONLY thing that H-D uses, so if you get brake fluid on the paint job, it won't 'eff it up.

IMO, it's the ONLY thing they've done right so far in a long time.

Al

A lot of years ago I switched my 1954 Reo/Swift heavy rescue truck (with a hydrovac) to DOT 5 brake fluid. While it was running (I can't get the engine started now!), I had no trouble with the brakes. People have said to expect some leaks, because it can get thru smaller spaces. It does not adsorb moisture as does DOT 3 & 4, so you won't experience corrosion throughout the system; however, I have heard that any moisture that gets into the system will settle in low places.

The changeover is not as simple as dump one and pour in the DOT 5. I honed all the wheel cylinders and put in new rubber parts, but you might have to sleeve them (or replace) depending on the amount of pitting. I also disconnected both ends of all brake lines and blew them out with an air compressor. Some people suggest flushing with alcohol. (An aside on the hydrovac: I couldn't (at that time) find someone to rebuild the hydrovac, so all parts were cleaned up and reassembled. It apparently worked OK.)

Do you know about ATHS (American Truck Historical Society) and ATCA (Antique Truck Club of America)? There's also a national society (SPAAMFAA) for fire apparatus if that interests you.

These are not all the details, but:

1) The system must be throughly cleaned of all the current brake fluid.

2) ANY rubber/plastic parts that you cannot verify as DOT 5 compatible MUST be replaced.

You may want to stick with regular fluid for that truck and occasionally suck out the fluid from the master cylinder reservoir and replace it with new DOT 3 or 4 fluid. Valvoline makes a synthetic DOT 3-4 fluid. It is a good idea to buy the smaller quantity bottle of fluid also, less chance of it absorbing moisture, which you would just pour into your truck's brake system negating your best intentions.

Beware the misplaced decimal point.......

The con would be someone putting incompatible brake fluid on top of DOT5. Other than that unlikely event, it's all good. I did a '54 Chevy and replaced all the brake lines (metal and rubber) and rebuilt all the cylinders. The master cylinder was replaced with a dual cylinder type rather than the original single cylinder. What prompted me to do this was the failure of the master cylinder and loss of hydraulic brakes. I had to get home that time with judicious use of the emergency brake and the manual transmission. The old brake fluid looked like black mud. Always check your brakes first when you bring an old car home. Those old cars are fun, but they are dangerous to you and others on the road if not maintained properly.

Just change brake fluid every 3 years. More often if you really don't want it to rust.