Break Fluid Type?

[White Spyder]

I need to change the break fluid in my 48 Chrysler. What is the best type to use in our old cars with oem systems?

[55 Fargo]

You most likely have DOT 3regular brake fluid, this is what you would want to replace that with.

If you are planning all brand new lines, hoses, Master Cylinder, Wheel Cyls, then you could go with DOT 5 silicone fluid.

Something you should do, is pump the brake pedal, periodically, if your car sits for periods of time, while your not driving it.....

[TodFitch]

Concur with C38Spitfire6: You want to use whatever is in there now.

 

DOT5, when new, has a purple dye. While mine has stayed purple for 20 years, I've heard that the dye can degrade over the years and the color go away. But DOT5 has nearly no smell while DOT3 has a very distinctive smell. Not sure about DOT4. So you can probably tell by that. If not, then DOT3 is a pretty good paint remover while DOT5 will not ruin old paint (just make putting new paint on a pain), so that is another test.

 

The most likely situation is that it has DOT3 in it now.

[desoto1939]

if converting from Dot3 to Dot5 Silicone then you also need to flush the entire system with denatured alchohol. As you do this and you capture the alcohol is a glass jar you will see that the two trype of brake fluid will sperate into two distict section. I did mine along time ago so I can not remember which fluid will be on top..

 

Flush the system at leaset twice.

 

Rich Hartung

[TodFitch]

if converting from Dot3 to Dot5 Silicone then you also need to flush the entire system with denatured alchohol. As you do this and you capture the alcohol is a glass jar you will see that the two trype of brake fluid will sperate into two distict section. I did mine along time ago so I can not remember which fluid will be on top..

 

Flush the system at leaset twice.

 

Rich Hartung

If I recall reading it correctly, DOT5 is less dense than DOT3 and DOT4 which means the DOT5 would be on top.

 

And that is one of the problems in converting to DOT5 - you will never get all the DOT3 out of the cylinder without disassembling everything as the bleeder is on the top. Apparently the interaction of the DOT3 and DOT5 in the cylinder over time is what causes the problem with the quick and dirty "just swap the fluid" method of converting. And I've read that flushing with denatured alcohol can cause issues with the rubber components...

 

Basically if you are going to switch fluid types you want to be doing it when you are totally overhauling the brakes and have all the cylinders open... Best time is if/when you are replacing all the components including all of the steel tubing.

[Chuck51631]

The Pro's and Con's.

When I used standard DOT3 brake fluid I was always replacing wheel cylinders due to rust or leakage every 2 to 3 years since the car was not driven much.

After replacing the brakelines, wheel cylinders, flexable brake lines and master cylinder (since I was doing a major brake job I converted to a dual master cylinder). Flushed they system with alcohol just incase the rebuilt parts had any DOT 3 brake fluid in them.

Filled the system with DOT5 brake fluid. It has been 7 years and no brake problems.

Every year I pull the brake drums and no leakage or locking wheel cylinders.

[55 Fargo]

The Pro's and Con's.

When I used standard DOT3 brake fluid I was always replacing wheel cylinders due to rust or leakage every 2 to 3 years since the car was not driven much.

After replacing the brakelines, wheel cylinders, flexable brake lines and master cylinder (since I was doing a major brake job I converted to a dual master cylinder). Flushed they system with alcohol just incase the rebuilt parts had any DOT 3 brake fluid in them.

Filled the system with DOT5 brake fluid. It has been 7 years and no brake problems.

Every year I pull the brake drums and no leakage or locking wheel cylinders.

With DOT 3, did you pump the brake pedal say a few times a month?It is said if the brakes are left untouched for longer time periods, say 6 months , they tend to develop problems.

[Niel Hoback]

  Back about 50 years ago, when driving these things was common, it was a regular practice to prop a stick between the brake pedal and the seat to keep pressure on the brakes.  It does tend to keep the cylinder cups spread out tight against the bores and prevent cylinder leakiage.  Fortunately, old guys then would share secrets with new drivers.

   Ah, yes, I remember. it was a first series 49 P-15 I got for $35. Had about 90,000 miles on it and was pretty tired. An old guy I worked with at the time was a Chrysler employee in the 20's and knew all sorts of oddities. Like the '30 Chrysler "6" I had was actually a '30 Plymouth with a different radiator shell.  Man, I wish he was around now,,,,,,,,,,,,,.