Gravity brake bleeding

[wrcsixeight]

I recently replaced my master cylinder and flexible front brake lines. In the process of removing the passenger side flexible line, I broke the steel line.

I bought some new line at Napa and bent it up using the proper tools, and used the old line as a template.

As I had no one to step on my brake pedal, and no mighty vac or similar device, I decided I would do a gravity bleed.

It worked fine on the back brakes, However on the front brakes, after 1.5 hours, not on drop of fluid made it past the bleeder.

Since this is on a Van, and I had the front wheels off, I was able to push the brake pedal with one hand and open and close the driver's side bleeder, but even after doing this, the fluid would no flow on it's own.

I enlisted the help of the cable guy to bleed the passenger side, then I did the driver's side myself again.

The brakes are firmer than before. I'm happy enough with the performance, however I am at a loss as to why they would not gravity bleed, even after The cable guy pumped the fluid through the new line.
The Dot3 fluid was warm.
The MC was in the sunlight. It was 85+ degrees
The Van was level.
The reservoir caps were not tightened.
The MC is about 2.5 feet above the bleeders.
The new steel line is not crimped.

I do have a hard time believing there is no air in the system, as the cable guy only allowed me 30 seconds of assistance, but the pedal engages the brakes slightly earlier than before and is firmer, and the thing just stops better.
I do have nearly a quart of new fluid left over. Since brake fluid does not really keep, I plan on bleeding them again with someone to push the pedal this time.

It just, why would the fluid not flow downhill on it's own?
Even when I used clean tubing and lung power to attempt to suck some fluid through, the air was just making it through the bleeder screw threads, and no fluid did.

The old fluid was only slightly dark for the first 8 inches. It was over 2 years old.

Ideas, opinions and humorous insults welcome.

 

[Torino]

I gravity bleed all the time. Yes, I change the brake fluid out yearly. Carry on. John--Las Vegas.

 

[oilboy123]

Sometimes I notice that getting the fluid to start I have to pump the pedal a couple of times or so.

 

[demarpaint]

I pumped the pedal about 5 times then opened the bleeder and let it flow out. When I collected about 10 ounces of brake fluid I closed the bleeder, went to the next wheel and repeated the process. It worked out fine for me.

 

[Quest]

Gravity feed brake bleeding will work properly only if you do not have air bubbles/pockets in the cavities to begin with.

It will not work properly if you have replaced your master cylinder recently.

Also: master cylinder replacement typically calls for 2-steps:

(1 ): bench purge the cylinder bores first with fresh fluid to make sure the cylinders bores are properly filled/purged.

(2 ): purge individual brake calipers/cylinders after you installed the master cylinder post-step-1.

Q.

I use either vacuum bleeding method or motive bleeder (pressurise master cylinder reservoir)

 

[bepperb]

I recently replaced my clutch master, and left the pedal depressed overnight to gravity bleed as had worked for others. Nothing. So now I own a mityvac which did the job in about 30 seconds. I'm not sure if I got some silicone grease that clogged the line, or if the lines were too thin, or if my aftermarket master didn't allow it to happen or what.

I have a one man brake bleeder cup you could make yourself for a dollar and it works well even with just me, if you're still open to non-gravity means.

 

[spasm3]

The key is to have bench bleed the master cyl per instructions. if you did not do this then you may have air in all the brake lines now! i would take it off and bench bleed it using the lines and adaper included. Every master cyl i have ever bought has instructions and plastic parts to do this. after you bench bleed it of all air the reinstall and bleed all the wheels starting at the right(passenger) rear then the left(drivers) rear then the right(passenger) front then left(driver) front.

 

[wrcsixeight]

I bench bled the new MC per the instructions. Mine only came with plastic caps to fit the outlets. I drilled holes in these caps and then inserted clear tubing into the drilled holes, installed the plastic caps tightly in the MC and inserted the clear hose into the reservoir. Tight fit, no drips.
I slowly moved the piston by hand a couple dozen times and watched the bubble move through the clear lines. I would then hold the piston and wait for any of the small bubbles to rise before releasing the piston, which would suck the bubbles back in. I did this a couple dozen times, until there were no bubbles, then walked away to do the brake lines. Every once in a while I returned, moved the piston and saw a few more bubbles purged. After about a dozen times of doing this there was absolutely no bubbles and it was hard to see if any fluid was even moving. I actually left it overnight because I needed more the steel brake line and Napa had closed. The next morning there was a few more tiny bubbles purged.

I replaced the old reman MC because it started leaking where it attaches to the booster. There had been no noticable loss of braking power. I replaced the flexible lines because they were over 15 years old. I replaced the steel line to the passenger side front, because I sheared it off while unscrewing the flexible line despite precautions..

Even after I had the cable guy press the brakes while I bled them in the normal manner, they would not flow with gravity.

I assumed there was still air in there, but the pedal felt firm, and a ginger test drive revealed the brakes worked better than before, and in the 11 days since, I've done a few really hard stops, and the brakes work better than they have in the 9 years I've owned this van, but they do not compare to my Dad's Audi, thats for sure. His rotors are probably bigger than my rims.

I'm just wondering why they would not gravity bleed even after I bled them in the traditional manner.

I'm wondering if the proportioning valve has something to do with it.

 

[oilboy123]

You can't compare brakes between different makes.

Heck you can hardly compare between two of the same cars. I have a 98 Ford Taurus and my Daughter has a 96. You would not believe how different they are from each other. They almost feel like two different makes.

 

[mechtech2]

Gravity bleeding IS pressure bleeding - atmospheric pressure.
[Just with less pressure than a device would produce.]

The bleeder may be blocked - try and remove it completely to see if flow initiates.
Also make sure your reservoir cap is loose to allow flow!

And finally, a Mity Vac or similar knock off vacuum device is wonderful for bleeding brakes. Well worth it and stone simple..
Just remember to keep your reservoir full!

 

[Carbon]

Originally Posted By: mechtech2
Gravity bleeding IS pressure bleeding - atmospheric pressure.
[Just with less pressure than a device would produce.

Not so. Pressure at the master cylinder is the same as at the bleeder for all practical purposes. Therefore it does not assist any more than it inhibits flow.

 

[mechtech2]

Originally Posted By: Carbon
Originally Posted By: mechtech2
Gravity bleeding IS pressure bleeding - atmospheric pressure.
[Just with less pressure than a device would produce.

Not so. Pressure at the master cylinder is the same as at the bleeder for all practical purposes. Therefore it does not assist any more than it inhibits flow.


I see your point about my choice of words.
But consider:
The fluid would not drain if the pressure was the same. It's called 'head pressure'.
So gravity bleeding IS pressure bleeding.

 

[SteveSRT8]

I gravity bleed both trucks and cars. Some will some won't.

It is a great way to purge old fluid, but I don't see how it does anything for brake feel or such.

 

[Silver]

Originally Posted By: mechtech2
Originally Posted By: Carbon
Originally Posted By: mechtech2
Gravity bleeding IS pressure bleeding - atmospheric pressure.
[Just with less pressure than a device would produce.

Not so. Pressure at the master cylinder is the same as at the bleeder for all practical purposes. Therefore it does not assist any more than it inhibits flow.


I see your point about my choice of words.
But consider:
The fluid would not drain if the pressure was the same. It's called 'head pressure'.
So gravity bleeding IS pressure bleeding.


Hmm. Would the atmospheric pressure at the bleeders not be even greater than at the MC, as the bleeders are at a lower elevation?

I think the explanation is, brake fluid is heavier than air, thus it runs down hill, or 'falls'.

Now before anyone gets all happy - I'm just joking around.