Brake Fluid

[Cob68]

Hi, just took the Cobra out for its first major run in what seems like 6 months, brakes were very spongy so blead the front brakes and seem OK now. I did consider changing all the fluid but is this a bit of an over kill? I might also bleed the back brakes but this is more of a pain.

[Richard]

great to hear the beast has been out for a run, when was fluid last changed and is it Dot5?

['The Gaffer']

Be good to see you out on the road again soon

This is what I found about checking brake fluid.

Step 1.  Locate the brake fluid reservoir, which should be clear plastic and labeled either on the reservoir or the reservoir cap. Wipe the cap with a rag before opening to avoid brushing debris, dust or other particles into the fluid.

Step 2.  Use a turkey baster or hydrometer to draw out some of the brake fluid for better visual inspection. Good brake fluid should be clear or slightly brown. It should closely resemble vegetable oil.

Step 3.  Notice if the fluid is dark brown or if there are particles in it. Over time, brake fluid absorbs dirt and moisture causing it to change and go bad. Brake fluid should be changed every 2 years or as needed, by consulting your vehicle maintenance guide.

Step 4.  Check with a professional mechanic if you are unsure about the life of your brake fluid. Take the vehicle to them and they will use the same methods of visual inspection, but with more skill and knowledge.

[Cob68]

Hi thanks for feedback, it is Dot4 as recommended in build manual, I have plenty so I think I will change anyway as can not remember how long its been in the car.

[Richard]

If I recall this correctly Dot 4 has a lower boiling point than 5 or 5.1.
Dot 5 is made of a different compound to 4 or 5.1 and absorbs air more easily but does last longer.
if you suffer with brake fade on more spirited runs it may be worth swapping to 5.1.
There may be a shelf life on brake fluid that is 4 or 5.1 but would need one of our experts to comment. As you cant recall changing it I'd swap it anyway.

[Hairy Santa]

Hi Tony,

brake fluid is very hydrascopic so if you are not sure -- change

wont take a lot of pumps to clear (change completely)

could try the mechanics trick of pump up the brakes then wedge the pedal down with a peice of wood and leave for a day or two

4 is ample for what your doing with the car

all the best

Pete

[Mark G]

The brake pedal should never go spongy just because of the brake fluids age.  The service life of brake fluid is due the the fact that it is a hygroscopic fluid, this means that it absorbs the moisture from the atmosphere.  This reduces the boiling point of the fluid which is why it needs to be changed.

The reason you have a spongy pedal could be one of many.  Normally its is due to having air in the hydraulic system.  If you have rear drum brakes then the most likely cause will be a leaking wheel cylinder,  this can allow air into the system and cause a soft pedal.  If you have discs all around then it could be down to a faulty piston or master cylinder seals.

On a separate note, I'd recommend to change all of the fluid in the system at the same time.  Not just front or rear as air can get into the reservoir and tiny bubbles can get into any of the lines.