partial brake fluid change

[michigan]

For the last few years, I have changed the brake fluid in the master cylinder only. I take it out with a syringe and replace it with the same volume. Am I accomplishing anything? I'm getting ready to do it again and am looking for some opinions on this technique. Granted, it's not all replaced in the system but, I believe that it is refreshed.

[coloradoporsche]

I don't think you are accomplishing much. You can buy a device to pressurize the system from Pelican and do a thorough job of it:

http://www.pelicanparts.com/catalog/shopcart/911E/POR_911E_BRKhyd_pg3.htm#item11

[Dan in Pasadena]

If you've got a dedicated partner you can just use the old fasioned foot pump bleed method. Get a clear bottle to pump into and of course change fluid color so you can tell when you've pumped all the old out and the new in. I agree with coloradoporsche, you aren't accomlishing much by mixing the new into the old.

[Gunter]

Due to heat factors, brake fluid deteriorates in the calipers much more than in the MC. All brake fluid contains moisture and that moisture (Water) ends up at the lowest point which is the caliper.
I am sure that some, or all, pistons in your calipers are frozen. Check and you'll see. If that is the case, a rebuild is in order.
It is recommended to flush the system with new stuff every two years to prevent the pistons from getting stuck.
Start with the right rear: Brake line-wrench on bleeder nipple, Clear nylon hose on the bleeder nipple into glass bottle with some fluid in it, open bleeder, leave open and pump pedal repeatedly filling new fluid into brake reservoir.
That's right! No need to open/close the bleeder as long as the hose stays submerged in the bottle-fluid, no air can be sucked back in through the nipple. (30 plus years experience)
After the right rear, do the left rear and the front the same way. Now you have fresh stuff in the system.

[911pcars]

While the "pump the pedal and open bleeder screw" technique is valid, there have been too many master cylinder failures due to excessive stroke of the pedal, especially by the uninitiated or overexhuberant spouse/girlfriend.

You can accomplish the same result by attaching a nylon hose from the hardware store onto the end of the bleeder screw (curve it upward, then down into a catch can), then open the bleeder screw. Gravity will cause the fluid to exit the caliper with perhap just a slight twitch of the pedal to get it going. Watch the reservoir as the fluid level drops, add fresh fluid before it empties. Watch for new fluid to exit into the tube, then close the bleeder screw. Repeat 3 more times. You should be able to refill the system with a single liter of fresh fluid, so use the best fluid you can (ATE, Motul, etc.).

Sherwood

[Dan in Pasadena]

Sherwood,
I had heard this worked as you describe I just wonder how long it takes for the full system to be purged and new fluid to start appearing in the drain container? Seems like one small twitch of the pedal would not precipitate a fast (or even moderate) rate of drainage. Also, by curving the hose upward coming off the bleeder screw that tends to negate the effect of gravity on the fluid in the MC, doesn't it? If the drain hose is placed in a moderate amount of fluid - not just placed in there dry, wouldn't that work in leiu of the upward curve? Just some ideas/questions.

[Wil Ferch]

No...it doesn't "negate" the effects of gravity since the MC is WAY overhead...it works on NET height differences..... basic fluid dynamics. The upward curve captures those nasty air bubbles readily......

As long as the MC is higher than the point of exit ( ultimate exit point)...you're OK...

Gravity method provides a nice high firm pedal...I'm a beleiver of this approach, although it is a bit slow, it is not terribly slow.

Wil

[911pcars]

Dan,
By installing the hose with an upward curve, this insures the bleeder screw is always exposed to fluid on both sides of its opening - thus no air can enter.

The curve of the tube should be below the level of the reservoir so gravity is always at work to push fluid through the system.

Yes, gravity flow is not as fast as pressurized flow, but one can exercise the pedal (not too far) to encourage fluid movement. Remember, since there's a tube filled with fluid on both sides of the bleeder screw, there's no chance of drawing air into the system.

You can also bleed more than 1 caliper at a time.

Sherwood

[michigan]

Gentleman, thank you for the interesting responses to my question on brake fluid exchange. I really like the idea of attaching a hose and keeping its flow out within the closed system principal. I personally will practice that in the future. I am now more inclined to do the job the right way with a full exchange of fluid. Thank you all.

[porschenut]

Why bother with any of those methods when you can do it easier and better with a $50 pressure bleeder? Nothing can beat it.

[Dan in Pasadena]

Dave,

Sounds like what Sherwood and Wil are proposing just DID beat it...and it doesn't cost $50! I suppose if I were in a hurry then thats another consideration. But if all I have to do is attach clear hoses, place them into clear bottles of fluid and wait for the exchange of new fluid to replace the old fluid, well....how much simpler could it get? Especially with zero chance of air entrapment. Sounds pretty idiot proof to me....which given my occasional lapses is a very good thing!

[911pcars]

I don't want to sound like a gravity bleed evangelist. If you have time constraints, then use any other alternate method of ridding the system of air and old fluid.

Harbor Freight sells a $4 brake bleed bottle. The kit contains various adapters and hose. If you look closely, you can see the air bubbles.



Sherwood

[porschenut]

Well, with a pressure bleeder you don't have to press the brake pedal, and you don't have to watch the level in the MC, and you don't need clear bottles of fluid. You just hook up the bleeder, give it a few pumps, and bleed each caliper like turning on the kitchen faucet without ever going back to the MC to check/refill or to push the brake pedal. Nothing to check or worry about, it's a one man job, and there's no chance of damaging your MC. Any of the methods are fine, so do whichever you feel most comfortable with. I'm just proposing an alternative that many feel is better.

[911pcars]

"Well, with a pressure bleeder you don't have to press the brake pedal, and you don't have to watch the level in the MC, and you don't need clear bottles of fluid. You just hook up the bleeder, give it a few pumps, and bleed each caliper like turning on the kitchen faucet without ever going back to the MC to check/refill or to push the brake pedal. Nothing to check or worry about, it's a one man job, and there's no chance of damaging your MC. Any of the methods are fine, so do whichever you feel most comfortable with. I'm just proposing an alternative that many feel is better."

Yep. Some of what you speak is true. Both methods eliminate touching the brake pedal, but you do have to watch the BF level, wherever it is; either in the MC reservoir or .... in the pressure bleeder tank. Using the PB tank as a supply reservoir for bleeding is both wasteful and unnecessary. How many liters of BF do you load up inside the pressure bleeder? What do you do with the excess fluid in the pressure bleeder? Pour it back into the container? Do you store unused BF in the PB or do you wash it out? There are those who advocate using the tank only to hold pressurized air. The fluid remains in the MC reservoir to minimize contamination. Either way, you watch how much fluid you have.

Gravity bleeding uses a minimal amount of fluid. I mentioned earlier the entire circuit can be filled with a little bit over the exact capacity. A pressure bleeder will tend to blow extra BF out the bleeder screw. However, YMMV.

Pressure bleeding is faster, but better is in the eye of the beholder. It's just another way to arrive at the same results.

Regards,
Sherwood