DOT 3, DOT 4, DOT 5 and DOT 5.1 Brake Fluid Use In Vintage Cars

By: Jack Boyle, with assistance by Charlie Pitts

This article is intended to provide you with the information you need to make intelligent decisions when selecting a brake fluid for your vintage car. This has been an ongoing issue in older cars for many years and yes, there are many and very vocal opinions coming from all sides of this issue.

The equivalent of DOT 3 brake fluid was used in our cars from the factory and for decades it was our only brake fluid choice. While we should all follow the manufacturer’s recommendations, technology marches on. Whether it is coolants, lubricants, nitrogen in tires, LED lights or brake fluid – times have changed and in some cases we can consider taking advantage of these technological advances.

There are now other options for brake fluid available to us so there is a decision to be made.  First a little background on the primary passenger car products.

DOT-3 brake fluid

DOT-3 (as well as DOT-4) is a tried and true glycol-based fluid. It is unfortunately hydroscopic which means it absorbs moisture, even from the atmosphere. For this and other reasons manufacturers recommend brake fluid changes every few years and many old car collectors already change brake fluid periodically to keep the moisture content at bay. DOT-3 and DOT-4 fluid will also damage, even remove paint it if left on the surface long enough. DOT 3 is compatible with DOT 4 but combined use is not recommended. In a new system or after a complete rebuild there would be no reason not to use DOT-4 as it has better performance than DOT-3.

Our original rubber parts in our old cars were formulated and manufactured specifically for use with DOT-3 glycol fluid and that was all that was available at the time. So while DOT 3 is very compatible with our vintage cars rubber parts as is DOT-4, its constant moisture absorption can wreak havoc on steel and cast iron parts such as fittings, lines, wheel cylinders, etc. especially when not exercised for extended periods. To address the moisture issue several manufacturers now offer DOT-4 LMA (low moisture activity) which, according to the manufacturers, reduces the moisture absorption which is important in seldom used collector cars.

DOT-5 brake fluid (a.k.a. Silicone)

Silicone fluid was developed over 40 years ago for use in military vehicles that were only used periodically and in many cases in humid climates (jungles). It is sometimes marketed as "lifetime" brake fluid. It is also considered by many old car collectors when you need to preserve brake systems such as in our vintage vehicles where their use may be intermittent. Another aspect of DOT-5 fluid is that it tends to retain air (not visible bubbles) in the fluid which may require multiple bleeding sessions. In some cases a spongy pedal has also been reported. DOT-5 silicone brake fluid is more expensive but is does not absorb moisture and it will NOT damage paint – however spills should be cleaned thoroughly and promptly. DOT-5 will not mix with DOT-3 or DOT-4 fluid and they must not be used in the same system. More about flushing later. 

Silicone (DOT-5) fluid may or may not be incompatible with the rubber parts in your old car’s brake system, for this reason numerous restoration shops and many rebuilders are strongly opposed to DOT 5 use in old cars. The primary concern is with the formulation used in old or older rubber parts in our brake system. In many cases seeping and leaking have been reported. DOT 5 fluid can also cause swelling of some older rubber parts. In some cases such as wheel cups, a small amount of swelling may not be a problem. Swelling in master cylinders could be more of a problem and in some cases brake lock-ups from stuck master cylinders have been reported.

DOT-5 fluid is being used in more new cars every year. In these cars, the non-metallic and rubber parts are formulated and manufactured specifically for use with DOT 5 fluid.

From actual hydraulic performance characteristics including the boiling point, DOT 3, DOT 4 and DOT 5 are all satisfactory for normal passenger car and light truck use.

DOT-5.1 brake fluid

To complicate matters even further, there is now another Fluid available, DOT-5.1. The difference is DOT-5.1, like DOT-3 and DOT-4, is a polyethylene glycol-based fluid (remember DOT-5 is silicone-based). All polyethylene glycol fluids (including 5.1) are hygroscopic and will absorb water from the atmosphere which is very corrosive. Basically, DOT-5.1 performs from a boiling point standpoint, as the non-silicone version of DOT-5. DOT-5.1 by definition must be less than 70% silicone. Above that 70% threshold makes it a silicone based DOT-5 fluid. Think of DOT-5.1 as a glycol based fluid that meets DOT-5 silicone performance standards – confused yet?

So where do we go from here?

These incompatibility issues can be hard to predict, observe and detect. In some cases discoloration of the purple appearing DOT 5 fluid can indicate deterioration or contamination. In the case of DOT-3, DOT-4 and DOT 5.1 old or deteriorated fluid can appear dirty, even opaque and rust colored.

If you want to read a more detailed discussion of the rubber and compatibility issue and fluid choices, I suggest:



Charlie Pitts, my co-author reminded me that he has been in the brake rebuilding business for over 50 years and has never had a booster come in for warranty work because of OEM fluid, but he has had several customers lose their brakes because of the swelling in the displacements type or leaking profusely with DOT-5. 

The safest bet for vintage cars at this writing seems to be DOT-4 LMA (low moisture activity) its formulation is consistent with vintage rubber parts and professes to absorb less moisture than our old reliable DOT 3. Regardless of the fluid that you use in your vintage car, fluid changes are still required to flush dirt or absorbed moisture out of the system. The frequency of the needed flushes are dependent on the climate your car is driven (or stored) in and your threshold for replacing all of the steel parts periodically.

In spite of this if you do decide to use DOT-5, you must completely flush the system of all glycol based fluids which in most cases requires disassembly and through cleaning. My dad (a 50+ year mechanic) then recommended blowing the lines out several times with rubbing alcohol and then blowing the lines dry before reassembly to make certain there is no remaining DOT 3 or DOT 4 fluid in the system. Once the new fluid runs clean, clear purple and is bubble/air free you should be good to go. You should monitor your fluid level and fluid color purity and clarity often.

My Experience

I have used DOT 5 in all five of my old cars ranging from 1937 to 1971 and have had no problems save one situation which is unique to 1955 Chryslers and Imperials and the primary reason for researching this article.

The 1955 (one year only) Chryslers, Desoto and Imperials using the Kelsey-Hayes power brake master cylinders were a very unique master cylinder design. This master cylinder is not typically used in Dodges or Plymouths. These master cylinders use a system where a rod slides through a series of “donut” shaped seals in the master cylinder and the fluid is displaced, not pushed forward. All other known master cylinders use a system where the fluid is pushed forward in a column by a piston sealed against the cylinder’s interior wall.

Recently my 1955 300’s master cylinder seals completely failed and apparently in a short period of time. The master cylinder was less than 8 years from a complete rebuild using new parts. The DOT 5 fluid was cloudy and the seals significantly deteriorated. There was also rubber debris in the fluid. This led to a total failure of the brake system – and not at a good time, but no harm done. 

I sent my master cylinder to Charlie Pitts at Power Brake Exchange, Inc. to be rebuilt. Power Brake Exchange is recommended by several clubs and club members and they have been doing rebuilds since the 60’s. Charlie recommends against using DOT-5 in any old car and in particular against its use in these 1955 units due to the potential swelling of the unique seals which seal against the cylinder housing and the moving displacement rod. 

Charlie assisted me with this article and suggests that we all go VERY cautiously into DOT 5 usage in all of our old cars with our eyes wide open to the issues and concerns. I for one intend to continue to use DOT-5 in my other, non-1955 old cars but I will watch the system for leaks and deterioration more closely than ever before.

Jack Boyle
Old Car Guy
Charlie Pitts CEO
Power Brake Exchange