3, DOT 4, DOT 5 and DOT 5.1 Brake Fluid Use In Vintage Cars
Jack Boyle, with assistance by Charlie Pitts
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.
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
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.
(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.
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.
brake fluid (a.k.a. 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.
(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.
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.
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.
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
fluid (remember DOT-5
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?
where do we go from here?
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.
you want to read a more detailed discussion of the rubber and
compatibility issue and fluid choices, I suggest:
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.
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.
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.
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
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
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.
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.
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.