FredR

As part of my current maintenance programme I was going to bleed the brakes today but then noted my bleeder vessel in the Mityvac kit was missing presumably after forgetting to clean and put it away after using it for something else!

Anyway a couple of queries:

1. I have a 1 litre can of ATE blue brake fluid- quite some time ago [3 years?] it was inadvertently opened [was not needed] and immediately resealed- none having been used so is more or less full to the top. Should this be considered suspect and ditched or should it be OK given it was not left open?
2. For bleeding is the consensus to start at the nearest point or the furthest point? I have read both approaches as being recommended but no position on tis matter.

Thanks in anticipation

Fred

123quattro

I would toss that fluid. As it's been opened it will absorb moisture.

Try and find a way to empty the reservoir. Then refill it with new fluid. Bleed any circuit long enough that you feel comfortable that the new fluid has made it to that circuit. After that bleed the remaining 3. Personally I don't see why it matters which corner you start with. You just want to swap new fluid into all the circuits and reservoir.

RFJ

I don't understand, how is it going to absorb moisture when it was closed right back up?

mark kibort

freaks get way too worked up about open bottles. the fluid would absorb the moisture in the air trapped in the can.. its almost nothing.. unless you are racing at the limit of the system, i wouldnt give it a second thought.
the fluid sits in the car for years, with no issue... (and its exposed to air constantly)
the can stuff is fine. Please dont worry about it.

blead from furthersest to closest caliper . use two man method. push slow down and slow up... (reduce chance of cavitation... air bubbles)

RFJ

Thanks Mark.

jpitman2

Unless you are in a hurry, if you dont have a helper, let the fluid fall under gravity, until you see the colour change come through the bleeder - less disturbance, less chance of over stroking etc. Just keep checking the reservoir .

jp 83 Euro S AT 56k

GUMBALL

If possible. bleed a front and a rear caliper simultaneously. By doing that, you get full pedal travel, and you ensure the "shuttle valve" stays centered.
If bleeding a caliper that has 2 bleeders, open the inner first, then close that and go to the outer.
Use a pair of "bleed bottles" (I use plastic 1 pint bottle, with a 6" to 10" clear plastic hose 3/16" ID) and keep them above the caliper so you can see bubbles or fluid change in the hose.

gcthree

Or, get a Motive Power bleeder with the correct adaptor for the 928 master cylinder. Be sure not to overpressurize the system and blow the blue hose off. Nothing works better.

ThetaTau87

Once the original seal is broken the container is no longer completely air tight and the fluid can absorb moisture that gets past the cap, especially over a long period of time. This is the same situation as when it's in the car. The brake system is sealed, but not air tight like an unopened bottle of fluid. Moisture migrates past the seals and is absorbed by the fluid.

Think about food that can sit on a shelf for years without going bad, but has a limited shelf life once the original seal is broken, even if the top is closed tightly. This is the same principle.

Most shops have a tester that can quickly and easily measure the moisture content of brake fluid. The best thing to do would be to take the bottle to a shop and have them test the fluid. If it was sealed up tight it is probably fine, but only one way to know for sure.

It's probably also a good idea to completely replace all the fluid in the system rather than just bleed the system a little. You may need more than 1 liter to completely replace all the fluid in the system. Assuming the color of the fluid currently in the system is gold, it will be easy to see when the blue fluid starts to flow out of the bleed nipple. This is precisely why the fluid was blue. By alternating between blue and gold fluid for flushes it was easy to see when all the old fluid was out. It worked great until the feds decided to enforce regulations that had long been unenforced that require all brake fluid to be a gold color and ATE Blue was banned.

jpitman2

Whats the small hole in the reservoir cap for - to allow AIR in to replace fluid consumed as the pads wear. There for its NOT sealed. If there was no air bleed the fluid would reduced pressure and stop feeding into the cylinder.

jp 83 Euro S AT 56k

FredR

Gents,

As always some interesting suggestions/thoughts- many thanks.

The main reason I want to use the blue fluid is because ATE is not readily available here and the blue is ideal for a fluid change to help one see the transition.

I suspect 1 litre would not be enough if starting from scratch with a pile of air bubbles to displace but I am simply doing a fresh fluid change as the current stuff is about 3 years old so providing I do not "***** up" the process I should be fine volume wise.

Unfortunately there are no testing facilities here that I am aware of so it is a bit of a crap shoot if I do use the blue fluid. I am tempted to use it just to prove I can do it satisfactorily- I then have the option of doing it again with freshly opened fluid. At least the comments prompted me to think a bit and I just took a look at the ATE can cap -it is a plastic nozzle and plastic cap- so effectively soft sealed- therefore doubt any air can migrate even after the original seal is broken.

On the subject of breathing in the reservoir cap whereas the level does go up and down as the brakes are worked, the insert of the cap has a concertina type diaphragm that [theoretically] motions with the level- the air breather allowing air in flow into the non contact side of the diaphragm- so my understanding is that the small amount of captive air is rapidly exhausted of moisture content that is not replenished during daily use.

I also noted during my research that some recommend putting a layer of sealing grease around the base of the bleed nipple to stop that being a leak path. No idea if our units need this - I generally just crack the nipple until fluid starts to come out and no more.

Many years ago in the UK I had a self sealing kit that you shoved over the nipple with a simple check valve device in the end to prevent back flow of air- seemed to work OK. I will probably use a tube plumbed into a collection jar and ensure that the tip is immersed in some fluid at the base of the jar to prevent any possibility of air back flowing.

Just need to get my wife or daughter to pump the pedal for me!

Rgds

Fred

928Myles

Fred,
1 litre is a bit light. The WSM states the capacity of each calliper at about 250ml and the system as a whole at about 1 litre. You would need to not have any mixing of the fluids in the calliper to bleed with only 1 litre. I used 1.75 litres when I did mine recently, but I wanted to be sure of a clear colour change so probably over did it.

Myles

FredR

Myles,

Interesting comment- sounds as though I may end up having to use something else!

I have no idea of the volume inside the calipers- all I can say is that I expected there to be little more than the swept volume of the pistons which does not really amount to a great deal but of course there are 4 pistons in each caliper so it all adds up.
It will be interesting to note if anyone else has measured the amount displaced by a straight swap.

Rgds

Fred

Mrmerlin

bleeding is done LF RF LR RR ,
the opposite of what was suggested ,
ref the WSM.
I would be concerned about the fluid if it was opened in a high humidity area,
if your area is more dry than humid,
then the moisture available may not be an issue for opened brake fluid thats has been capped.

The only caveat is that if the MC is old then the chance is increased that the bore has corrosion on the unused section of the bore,
and thus as the pedal is made to go full stroke the seals cut into the corroded portion of the bore,
thus causing them to fail.

If the pedal was good then after some bleeding cycles you lose the pedal then you may be in for a new master

jpitman2

When did 928s change to a master cylinder with an air isolation diaphragm/bellows arrangement? I would not have thought there was room for such an item in mine.

I last flushed my fluid with a mightvac, single handed, and the only issue was pulling the whole reservoir off the cylinder instead of just the cap! AAARRGGHHHH! Panic stations!
jp 83 Euro S AT 56k