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The story.

Car: NA6 all original brake hardware (ie not modified)

Brakes were working fine. I noticed that under the rear driver side caliper it was leaking a little brake fluid. Nothing major, but I wanted to stop it. So (to keep things symmetrical) i got two rear seal kits, with new caliper pistons and slider pins and completely rebuilt both rear brake calipers.
Everything looks fine up till this point.

Now i am in the process of bleeding my brakes. I followed the instructions here: viewtopic.php?f=76&t=37622 which seem to work great. Except there is one problem, the air bubbles coming out of the clear 'drain' tube don't seem to stop!!! I havent bled brakes before but have read lots of other instructions and (although not a pro or anything) I am not a complete idiot when it comes to mechanical things. I have been gravity bleeding the brakes for about 2 or 3 hours all up now with the occasional pump method every now and again.

Is this normal? Given that when i rebuilt the calipers they (and the rear brake lines) were COMPLETELY drained, i'd expect there to be more air than a straight fluid change, but the air bubbles never seem to cease! I see no evidence of a leak anywhere, so i can't for the life of me figure out what is going on. The endless air bubbles are the same on both rear brakes. Even under pressure (when if there where any leaks air would not enter but rather fluid would get forced out any holes) the air just keeps coming. There are periods where there is no air, but then after a while it just keeps coming like i've hit another air pocket or something.

Does anyone have experience bleeding brakes from a fully dry system or anything like that?

Any help would be greatly appreciated as i haven't been able to drive my car for over a month due to waiting for parts etc!!!! and it's driving me crazy.
Cheers.

also if it helps explain.

if i close all the bleeder nipples and depress the brake pedal there seems to be good pressure/feel solid. ie the brake doesnt feel spongy or anything.

I assume that you have not allowed the brake fluid resevior to become empty during the bleeding process? A dry resevior will allow air into the system.

You mentioned that you had rebuilt the rear brakes. Could the front brakes be allowing air into the system?

J

I just had a read of the instructions you referred to. I have not personally used a 'gravity drip' bleed method, I have used the either the 'pump and hold' method requiring two people, or a bleeding tool with a one way valve that does not allow air into the hose when the brake pedal is released. I currently have speed bleeder valves on my car that have the one way valves built in.

I noticed in your post that you mention that you have pumped the brakes a couple of times. Was the bleed valve closed when the brake pedal was allowed to rise. If not, air is probably being sucked back into the system via the hose.

Hope this helps in some way.

J

Juffa wrote:I just had a read of the instructions you referred to. I have not personally used a 'gravity drip' bleed method, I have used the either the 'pump and hold' method requiring two people, or a bleeding tool with a one way valve that does not allow air into the hose when the brake pedal is released. I currently have speed bleeder valves on my car that have the one way valves built in.

I noticed in your post that you mention that you have pumped the brakes a couple of times. Was the bleed valve closed when the brake pedal was allowed to rise. If not, air is probably being sucked back into the system via the hose.

Hope this helps in some way.

J

Whenever i have 'pumped' the brakes it has been with two people and i have only released the bleeder nipple when the pedal is depressed.

After another few hours bleeding I now suspect that the air is entering from the thread around the bleeder nipple! I don't beleive that it will bring in air when it is tightened, ut rather when it is loosened to let fluid out, the conical tip doesnt seal, and lets air in through it relatively loose thread.

As the brake pedal remains solid when depressing, it think i will test the car out tomorrow, see how it goes, and take it from there.

Juffa wrote:I assume that you have not allowed the brake fluid resevior to become empty during the bleeding process? A dry resevior will allow air into the system.

You mentioned that you had rebuilt the rear brakes. Could the front brakes be allowing air into the system?

J

when you say dry, how low does the fluid have to go to get dry/let air in? In all the hours i have been bleeding, only once has it gotten about 1/2 between the low and max levels. its almost always completely topped up.

That would be OK.

You're probably correct about air getting in through the bleed screw. It only has to be just cracked open (1/4 of a turn max) if pumping fluid.

If your using a clear bleeder plastic line I think you find its not sealing properly around the top of the bleeder, allowing air to enter the hose but not the brake system. Try bleeding without the hose and allow the fluid to gravity bleed, but put a container under the disc to catch the fliud, saves time cleaning up. When finished and you have locked of the bleeder, firm not too tightly, wash down with water.

Terry

But if you do that how will you know when all the bubbles are gone??

I think its just a 1/4 turn like just loosen the bleeder to bleed it.

I've had endless bubbles when the seals in the master cylinder were gone...

Nark wrote:I've had endless bubbles when the seals in the master cylinder were gone...

Good point, does the car suffer from pedal fade?

it sounds like all the other potential problems have been ticked,

a few main points.....
make sure your not undoing the nipple too much, like the others have said 1/4 turn is heaps then make sure you lock it before you release the pedal again

check that the hose is on the nipple firmly & correctly & not sucking in air

get someone to pump the brakes a few times (make sure they have a firm pedal) & jump on them, look for small leaks around the braking system (not just the area you have worked on)

then check the master cylinder

also ive been told never to let anyone try & do brakes unless they have someone experienced with them, i know it all sounds easy & all but you are playing with the most important part of your car & could lead to a dangerous situation so please be carfull testing them....

I reckon Terry is probably right. The other thing to test is simply check ( leave the car and make a cup of coffee ) to see if the pedal is spongy. If the pedal only comes up with a pump then you have air in the system.

I'v bleed my brakes a number of times (i'v always serviced my own cars but not a mechanic) and always start the process from the master cylinder then move on to the longest brake line and so on till i get to the shortest, i also do it on my own with a glass container half filled with brake fluid and 2 holes in the lid, (one hole with a clear hose sitting in the fluid and the other on the bleeder nipple) the idea is you can do it yourself with the nipple open between 1/8 to 1/4 turn open so when you pump the brakes you bleed the air and when you let the peddal go it should suck in the brake fluid.
Its also advised not to push your brake peddal all the way to the floor as it could damage the rubber seals in the master cylinder which might be why your getting alot of air bubbles..

Hope this helps

Craig