Gravity Bleeding: How?
 

I've read the posts on bleeding (there are a lot): Motive pressure, vacuum, gravity. Current status of brake system: New MC, new rubber lines all around, rebuilt calipers installed bleed screw up. Did not bench bleed MC. Reservoir full, no drips sitting overnight, no drop in fluid level in reservoir. I'm fairly certain that the system is tight. (I'm okay with being wrong.)

Please explain the gravity bleed method.

Right now the pedal has no resistance although the reservoir is full of fluid and I've used the vacuum bleeder all around. It pulls air but little liquid.

From my experiences bench bleeding the master cylinder makes bleeding quite a bit easier.

As for pulling more air than fluid, you might be sucking air from around the bleeder - hose or threads on the bleeder itself. My vacuum frequently has some bubbles.

Gravity bleeding I haven't done but there are those who say to open bleeders and let fluid drain on its own. It seems like a long process.

Personally I prefer the old two man pump the brakes method.

i have used this method.it worked well.
keep the reservoir full. start at the furthest point.
just a slight drip is required. it is time consuming.
maybe an hour per bleeder?
bleed into a container.

Bleed the Master first. If you didn't bench bleed, you can still do it. Loosen the fitting from the m/c to the brake line(s), depress the pedal, and then tighten fitting before letting the pedal return. Repeat until the air if out and just getting fluid thru.

Then, move to the caliper the most distance away (right rear) and bleed again by whatever method you prefer.

Here's another alternative:

http://forums.pelicanparts.com/porsche-911-technical-forum/427910-5-pressure-bleeder.html

http://forums.pelicanparts.com/uploa...1390308955.jpg

I have used it MANY times on MANY cars with no issues.
The procedure changed slightly. Can bleed brakes in about an hour.
I set the regulator so it just barely pushes air out the hose.
Kinda gravity plus bleeding.

I've used the following process for the last twenty years, three or four times a year after track events.
Many of you, especially those with early less complicated cars, can bleed your brakes easily in less than 30 minutes without an assistant, without spending more than $5 on equipment, and without causing any undue stess on the master cylinder. My 74 911 lacks a hydraulic clutch so I don’t know if this procedure can be adapted for use with non-mechanical clutches.

You will need: brake fluid, six or seven feet of plastic tubing sized to fit snugly over the nipple on the brake caliper (mine uses 3/16” internal diameter tubing but check yours), and an appropriate small box wrench to open & close the bleed valve. (Sears has small 6 sided combo wrenches which are useful for recalcitrant cases; the bleed valves on my car are 7mm and 8mm but some are 9 I believe. .)

Here’s the procedure: jack the car up, support it on jack stands or wood blocks, and pull the wheels so you can get at the calipers. Or do one wheel at a time if you want.
Loosen the brake fluid container cover in the trunk.
At the first wheel, fix the plastic tubing so it hangs straight down (from your garage roof or door or even a stick or spouse) to the caliper. Slide the box wrench over the nipple, slide the plastic tubing over the nipple, and open the nipple with the wrench.
Watch the fluid rise in the tubing until it reaches the height of the brake fluid reservoir in the car, maybe 15 inches. This will take 3-4 minutes. Tap the caliper a few times with a rubber hammer or block of wood if you want. Watch the tubing for bubbles, especially when you start. A flashlight held behind the tubing will reveal all. If the bubbles keep on coming after the level in the tube has risen 12-15 inches, close the nipple, remove the tubing, drain the fluid in it into a paper cup, and repeat. Assuming you are bubble free after 12-13 inches of fluid have drained, however, do this once; tighten the wrench, pull the tubing off while holding a cup to catch the fluid, and move on to the next wheel.
Do all four wheels, keeping an eye on the fluid container and replenishing the brake fluid when it's down more than an inch.
You are done. Gravity has done all the work. You cannot damage your master cylinder with this technique, and you don’t need a helper (if you have a garage or a tree or can rig a stick to hang the tubing from.) The process should take you a maximum of 5 minutes per wheel once the car is up and the wheels are off.
Forget the idea that there are bubbles all thru your lines and that pressure will somehow push them out but not the fluid. A pressure bleeder only moves any bubbles and fluid thru the lines more quickly. But they travel together. So gravity will do the job perfectly if time isn’t the object.
Forget also the notion that the pressure generated by your brake bleeding kit will somehow give you a harder pedal. You can’t increase the pressure inside the system any more than gravity allows; if you could the fluid would compress when you applied the brakes and reduce their effectiveness.
This process works well because nearly all the bubbles generated by driving around and heating the brakes excessively are found at the very top of the caliper near the bleed valve…where they concentrate when generated by heat. (Any 5 year old child knows bubbles don’t move down in a liquid, so they don’t flow down to the bottom of the caliper then across the hard lines and back up to the master cylinder. If they did, they’d escape from the master cylinder reservoir into the atmosphere anyway.)
Depending on how much fluid you withdraw each time, you will find that the fluid is fully replaced every 10-15 bleedings. I track my car 5-6 times per year; bleeding pretty much replaces all the fluid over a couple of years, as recommended by many experts.

(Get the air out of MC for starters - find a friend and put this to bed)

As noted - Sucking air from around the hose at the bleeder is normal with the hand held mityvac.

For $20-ish you can buy bleeders with one way check valves. Once you loosen them a bit, they will allow you to use the foot pedal method solo.

Rambling on here: One of the downsides to the pressure bleeder and gravity methods is the lack of brute force applied relative to the foot method. The downside is time - NOT QUALITY - of the bleed in the brake lines. With your foot you can squirt brake fluid across the garage and given enough fluid refilling the MC as you pump, you could probably get the air out of the MC. I am pretty sure the other methods will not agitate the air enough to force it to leave the MC.

Nice write up.

I wonder how much fluid, not counting the cylinders is actually in the lines? A cup maybe?

Don't I remember that you use about a pint of fluid when you start from scratch? Anyone done this recently?

Moneymanager has it right, I have only ever used gravity bleeding and would add the following to the procedure.

If you are doing a new system or using silicone fluid (which I use and love), you need to move the bubbles through the line that goes from front to rear. Easiest way to do that is to set up as moneymanager described, but lower the farthest corner below the M/C level and let a fair amount of fluid flow, enough so that any entrained bubbles will move through the lines, out through the calipers and you are left with nothing but fluid.

The process moves slow enough that it is easy to just shut off the caliper and refill the M/C reservoir as needed. Repeat the process from farthest away to closest (although after you have done the first one you don't need to flow much fluid as you are only taking entrained air out of the branch brake line and caliper).

Did that back in 1998 and haven't touched it since. The joys of silicone Dot 5 by the way....I bought 3 pints, still have 1 left.

Dennis

Moneymanager: great description of the method. Thanks.

I haven't tried that method, but what bothers me about it is that when I'm inside pressing the brake pedal, I can't see the bubbles coming out of the caliper and therefore don't know when I've pushed enough fluid through it. Or how much I've pushed through it.

Porschenut, I don't think that you pump the pedal with the aforementioned method. You let gravity do the bleeding.

Yup, no pedal pumping (which in some cases can damage MC.)

I think the check balls require force to open them, if that is the method you refer to. With gravity bleeding they are not necessary.

However, I only used them once so not absolutely sure. No quality difference vs other methods.

Pedal Resistance
 

Gravity bled all four calipers. Fluid filled the line.
Grand daughter was getting fussy.
Had Daughter-in-law sit in driver's seat to bleed brakes with granddaughter on her lap. (Age and wisdom beat youth and energy every time!)
Air came out of each caliper.
Brakes feel great.
I love my granddaughter for being fussy and my daughter-in-law for, well, pumping the brake. Ha! Cheers!

The Porsche is back on the road sometime this weekend.
Here in Seattle we're above freezing, no rain.
Eat your heart out east coast.
And the SeaHawks will kick Denver's butt!!!!!

FWIW, when i pressure bleed my brakes i use about 200-250 ml per cylinder.

Bob, as you can see from my post, I was quoting your description of using the brake pedal method with the speed bleeders. THAT is what I'm talking about. I'm not talking about gravity bleeding.

Moneymanager: Thanks for the thorough description. Two questions on your method:
1) Why do you need so much plastic tubing? Wouldn't three or four feet work as well, as long as the top of the tube was higher than the MC to avoid spillage? I'm thinking I can use some blue tape to hold the (clean) tubing to the fender while I wait for the fluid to seek it's own level;

2) In order to do a brake system flush, could I use the same set up, but drop the end of the tubing down into a receptacle for the old fluid while I wait for the new, clear fluid to emerge?

I've worked on cars for decades, and never heard of gravity-bleeding until joining this forum. I'm itching to try this out now...

GK

I just completed the process.
Three or four feet of tubing is quite adequate.
I fastened the tubing to a ladder away from the paint. Don't get fluid on the paint!
Yes, the tubing can be lowered to allow fluid to drain until new fluid is seen.

Following a gravity bleed, I still had another person pump the brake pedal.
Air came out of the bleeder screw at each caliper.
So, gravity bleeding does a good job of moving fluid quickly through out the brake system but it does not remove all the air.