Fresh brake fluid

[Mark Haymond]

Over the years I have repeatedly read that it is good practice to use fresh brake fluid when servicing a brake system because an opened container of brake fluid absorbs moisture. The moisture can collect in low spots in your car and cause rust. And I have replaced a few wheel cylinders and master cylinders with rust damage.

Our cars have master cylinders that are vented to the atmosphere, often a humid atmosphere. Did the designers know about this in the 1940's?

[Don Coatney]

Our cars have master cylinders that are vented to the atmosphere, often a humid atmosphere. Did the designers know about this in the 1940's?

I would like to think that they did know about this. Any good engineer would take this into account. But much like today they accepted this potential failure as a means to sell parts and service down the road.

[Plymouthy Adams]

yep

todays systems are vented to the atmosphere..some in a similar method as old but traction control require a bit more 'venting'

[Adam H P15 D30]

yep

todays systems are vented to the atmosphere..some in a similar method as old but traction control require a bit more 'venting'

No, they are sealed. There might be a vent hole in the cap but all the ones I have seen have a rubber diaphram between the cap and fluid.

[Rusty O'Toole]

It was one of those ideas that seems obvious after someone else thinks of it. When the first cars were invented it was an achievement to get one to go 500 miles without breaking down. What might happen 5 or 10 years down the road did not even enter into discussion, in fact they would have been surprised to find their cars lasted over 10 years.

[Plymouthy Adams]

fill your master to the brim...take the car for drive and heat the brakes a few hard stops..see what comes out of the master due to expansion of now hot brake fluid..now how did that fluid escape if not vented..

[Young Ed]

You will also see this if you always check and top off the fluid as your brakes wear. When you go to replace the pads and compress the calipers the master will overflow.

[Mark Haymond]

I just thought it was odd that maintenance instructions said I should use brake fluid from a freshly opened container, pour it into a master cylinder open to the air where it quickly turns dark and brown, and just check the level once or twice a year. And then if I needed to add fluid, not use my tightly sealed partial container from my garage shelf to add to the old system fluid. Huh?

Water moisture really does get into the system. So I have concluded it is good practice to bleed the brakes and add all new fluid once a year to get any water out and prolong brake parts life. Anyway, I feel better when I see the dark stuff coming out of the bleed valves and the new stuff taking its place.

[Plymouthy Adams]

the manual and the person who wrote it followed the lead of the engineer..they have no idea when the brake fluid was opened or it sat for hours before the owner sealed the can or if he poured just what he needed and immedately sealed the container. As you are more than not working behind only your own previous actions..the call is totally yours based on your own work habits.

[Scruffy49]

As long as the can was closed the fluid can only absorb what was in the airspace to start with. I opened a can from the 60s the other day and it was still a nice color. I would use it in my 69 D100 that doesn't leave the farm without giving it a second thought.

My really old truck will get all new brake parts and DOT 5 fluid. And all stainless lines, including the flex hoses. The fluid type/age is irrelevant as long as it is clean and the system is in good order.