Comparison of Dot 3,4,5 and DOT5.1 brake fluids

[desoto1939]

As many of you go to your local auto supply store and might see various types of Brake fluid bottles, that range from DOT3, 4, 5 and Dot 5.1.

 

I use DOT 5 silicone brake fluid in my 39 Desoto. I asked the counter person is Dot 5 and Dot 5.1 compatible and they could not answer the question.

 

So i did some searching and found out the DOt3,4 and 5.1 are glycol based products and can be used together BUT DOT 5 is silicone based and you CAN NOT mix dot 5.1 with DOT 5.

 

I have attached the document for your reading.

 

Rich Hartung

Desoto1939@aol.com

 

 

 

Brake fluid Dot 4.docx

[Doug&Deb]

Thanks as always Rich. You always provide great information. Do you have trouble with spongy brakes? I used Dot 5 in my last car and could never get the brakes bled properly. They worked fine but the pedal never felt right.

[desoto1939]

1 hour ago, Doug&Deb said:

Thanks as always Rich. You always provide great information. Do you have trouble with spongy brakes? I used Dot 5 in my last car and could never get the brakes bled properly. They worked fine but the pedal never felt right.

Doug:  According to my mechanic DOT 5 is a heavier brake fluid. I recently had my entire brake fluid bleed out of the system and new front wheel cylinders installed.

 

According to my mechanic these older cars used internal brake cup expanders attached to each end of the wheel cylinder spring and the expander then fit inside the rubber cup to keep pressure on the rubber cup.

 

He used some of these in my front cylinders.

 

I have a good firm brake pedal but you also must remember that there is a certain amount of pedal travel and this is documented in your repair manual the specific amt of travel.  If someone move the adjustment rod in or out that will affect the feel of the break pedal.

 

Also what affects the brake pedal is if the shoes are arced correctly to match the drum. each drum and set of shoes has to be done seperately just incase if each drum has been turned down. Also check the adjustment on each drum.

 

Also remember that you are dealing with older non automatic brake adjusting shoes and not modern shoes that are self adjusting.  So every 6 months in the state of Pa when these cars were getting inspected the mechanics would also adjust the shoes.  We do not put that many miles on our cars but we still need to see to it that we keep the brakes adjusted properly.

 

Hope this is helpful.

Rich hartung

Desoto1939@aol.com 

[desoto1939]

Today I went to the AutoZone and looked at the back of a bottle of 5.1 brake fluid.

 

In very fine print is shows that dot 4 and Dot 5.1 can be mixed. It did not state anything about Dot 5  so always read the back of any bottle of chemicals

 

to make sure it is compatible with other fluids in your modern or antique car.

 

I talked to the sales and also manager in the Autozone to inform them of the non compatibility of Dot 5 and Dot 5.1.  They even did not know this information.

 

So buyer beware at all times.

 

Rich hartung

[Sniper]

I recently helped by son flush the brake fluid on his BMW. They specify a low viscosity DOT 5.1 fluid.  According to what I read the low viscosity was a requirement for proper traction/electronic stability/ABS function.  Nothing we need to worry about in our cars but as Rich says buyer beware.  Don't assume parts is parts, lol.  the BMW spec stuff will work in our stuff, but DOT3 will not work in the BMW.

[desoto1939]

Sniper that for the update. reading the owners manual in a modern car is now so important and most people do not take any time to read it an or even dogear a specific page with fluid information to help maintain their cars.

 

Rich Hartung

[Plymouthy Adams]

the store managers/counter staff today typically NEED year make and model for fit/fitness as they have zero hands on experience....rather frustrating for those that know what they are looking for and specify this often carrying in the part number to be 'raked over' the make model and years questions....only the computer seems to know when in the store as the sales staff can rarely tie their own shoes.  Refreshing however to talk to someone at the counter who has insight.  

[Sam Buchanan]

1 hour ago, desoto1939 said:

I talked to the sales and also manager in the Autozone to inform them of the non compatibility of Dot 5 and Dot 5.1.  They even did not know this information.

 

Most likely they don't care...................see above post about being a slave to the database..........

Edited March 6 by Sam Buchanan

[DodgeM37]

Dot 3, 4, and 5.1 are all glycol-ether based fluids. The difference is the boiling points - higher number, higher boiling point. The down side to these is they are hydroscopic; they absorb moisture. Which can cause rust and corrosion.

Dot 5 is a silicon based fluid is is not hydroscopic. The Military uses this fluid since vehicles can sit unused for long periods of time. However Dot 5 can absorb air if not stored properly and can cause a weak petal.

The two fluids are not compatible and when changing from one to the other the system must be thoroughly cleaned or bad things can happen.

[Dan Hiebert]

I use DOT5 brake fluid in the D24.  I would call the brake feel "mushy", but I have good braking action.

[desoto1939]

17 hours ago, Dan Hiebert said:

I use DOT5 brake fluid in the D24.  I would call the brake feel "mushy", but I have good braking action.

Dan I have to agree with you posting about the mushy pedal. I also read that Dot 5 is a softer brake pedal feeling and takes some getting use to this type of pedal. I have a good pedal in my 39 Desoto also.

 

Rich Hartung

[Dartgame]

something I've noted about DOT 5, is it can often be troublesome to get air bled out, and if you have a leaky fitting it won't "self seal" like the glycol based versions. By self seal I mean corrosion on the fittings if weeping a little. Moisture will corrode the offending parts with time and stop oozing....but I prefer the DOT5 for its non corrosive qualities. As mentioned above if a vehicle is left unused, nothing much changes in the brake system.

[Sniper]

I wonder if you put a vacuum on a brake system that has Dot 5 in it could you get the air out of the system?

[Dan Hiebert]

I had completely rebuilt the brakes on the D24 before I began using the DOT5.  I ensured all the fittings were as tight as possible, so I wouldn't have any "micro leaks".  I wasn't thinking bleeding problems, but there were some seeps in the original system that really annoyed me, and I took this as the chance to prevent that from happening again.  I slowly and methodically bled the system using both a vacuum draw from the cylinders, and the good old fashioned pumping the pedal method, to the point that if I had been working for a brake shop, I would have been fired for taking too long.  It seemed more challenging to get the system bled right with the DOT5.  The main reasons I used DOT 5 had to do with the car sitting idle in occasionally heated garages during long cold winters in humid climates, since the DOT5 doesn't draw moisture; and that it isn't (as) corrosive in case I did get leaks.  This is going on 20 years ago, other than the mushy brake feel, I have had absolutely no problems with the brakes, no leaks, no weeping, nothing.  I haven't even needed to add any brake fluid in that time.  Our garage is constantly heated here, but the car still sits idle for at least 6 months every winter, and Spring can be humid with wide temperature variations, so I think I'll be sticking with the DOT 5.  That said, I don't use DOT5 in any of the other cars, they are driven much more often.    

[Sniper]

You now, with the Motive power bleeder I bought, it takes more time to put the car up on jack stands than it does for me to bleed all four brakes, by myself. 

 

 

[Veemoney]

I switched from glycols and I'm running the DOT 5 fluid in my 34 coupe with disc front and drum rear as well as my 47  WC with stock drums and master. I have a solid high firm pedal in both. May be just me but I think one of the keys is to limit as much agitation to the fluid when bleeding as possible to reduce air entrapment. No shaking the bottle or actuating the master during bleeding any more than needed. I use a generic power bleeder similar to the Motive unit Sniper refers too. Not sure how much it helps but seems to work for me.

• I fill the power bleeder the night before to allow the fluid to settle in the bleeder reservoir.
• Hook the power bleed system up and pump up to 5-7psi (lower PSI than the power bleeder instructions identify to use)
• Open the power bleeder valve and check for leaks in the vehicle brake system or the hook-up and fix or move forward.
• I slowly push the brake pedal down and hold for a few seconds and then slowly allow it to return.
• Open each wheel cylinder 1 at a time as you would normally to allow the air to escape.
• The next morning after allowing the fluid to settle I repeat the process to remove any remaining air that may have been entrapped and settled.