The Stovebolt Site Forums The Shop Area Engine Shop Dot 3 vs. Dot 5 Brake Rebuild

I am about finished with a brake system rebuild for my '49 3/4 ton truck, which now includes a new master cylinder and all new wheel cylinders. I've read up on the differences between Dot 3/Dot 4 (glycol) fluids (annual fluid replacement recommended) and Dot 5 (silicone) fluid (less water absorption and lower maintenance). Two questions: (1) Has anyone had a good experience converting to Dot 5? (2) Is brake cleaner the right choice to flush the brake lines, which are otherwise in good condition and have not been replaced (since I understand that alcohol is incompatible with Dot 5 silicone-based fluid)? All thoughts gratefully received.

If it were me I would go with the dot 3 but that is just what I would do. If you had replaced everything including all rubber and steel lines then I might (probably not) suggest Dot 5. However you didn't mention that you had replaced the flex lines and if you haven't stay with Dot 3 or expect problems. Just my 2 cents.
Good luck!

I just finished a complete rebuild on my 53 3/4 ton, used dot 3. It's cheap, easy to find and has worked just fine in this old truck for past 50 plus years. As stated, blead the thing with fresh fluid every year, you'll be fine. Oh BTW, I made a pressure bleeder from a 1/2 gal. garden sprayer, makes the job painless, blead the truck inside of 10 minutes by myself.

I changed to Dot 5 in a rebuilt brake system. I was told the brake pedal was less firm using Dot 5 but I can't tell any difference. My single experience has convinced me to change outthe other rigs to Dot 5 as I replace the brake lines. I don't know what cleaner is compatable with Dot 5.

I just rebuilt my brakes also but I went with dot4 from napa. Man where those wheel cylinders full of crap do not think they where ever done. The only problem I have is getting the rear break line male nipple out of the rubber hose. Thinking about putting some heat to it. Oh its a 59 3600 fleetside police truck in Vernalis Ca population 400 Sure do not minde only going 55mph in this farming area.

I used DOT-5 in a full-system rebuild in my '41 De Soto about 7 years ago.

The only real issue I had was that when I went to bleed the system, I used my 1940's pressure-bleeder tank (thoroughly cleaned-up and dried to remove DOT-3 residue).

The DOT-5 container (Cartel brand) didn't give any warnings about Silicone's tendency towards "air-entrapment"...when I put the air on the bleeder tank, I created a DOT-5 "Frapp`e", and then couldn't understand why I still had spongy pedal after bleeding the system five times!

After giving-up and letting the car sit for a few days, I called Cartel's tech support and asked them what was up with this, and after going over my situation for about 15 minutes, the tech zeroed-in on the pressure bleeder and figured-out what happened. He suggested slowly and carefully pedal-bleeding, using the time-honored two-man method, and when refilling the Master Cylinder, to pour slowly so as not to "glug" or splash the fluid and create air bubbles. If I did see bubbles in the reservoir, I was to wait until they percolated-out before going further.

The second approach worked, and the brakes have been fine for the last seven years / 17,000+ miles.

I have gone back to DOT-3 or 4 in subsequent rebuilds.

So, if you go DOT-5, my suggestion would be to avoid the pressure bleeder, and be carefull not to "aerate" the fluid.

I have also heard other guys talk about DOT-5 allowing localized pockets of water to collect in the system (as opposed to DOT-3 becoming water-logged through-out). These pockets can cause localized corrosion, become vaporized during heavy braking, or turn to ice in freezing weather...

Thanks to all who responded. I went with DOT 3, and the old two-person bleed and I now have brakes that actually work. No more phoning ahead to put out the hay bales. Life is good. Now all I have to do is finish the rebuild on the old 216, stuff it back in there, and we'll be good to go.

Rhino,

I'm getting ready to resurrect Grandad's '54 for the fourth time...I rebuilt the entire brake system around 1983...

I will do everything, if necessary, but at any rate, am planning on DOT-3 or 4.

Good luck with yours !