How to unblock brake line.

[Okedokey]

I flushed the brakes last weekend, and afterwards, during the week the brakes locked on. I scanned and it came up with the brake pressure sensor and/or brake switch being the likely candidate for the codes. I have a new sensor, but while I was waiting for that I decided to re-flush the brakes. Starting at the rear-left (furtherest from the reservoir), I couldn't get any fluid out of the caliper when pumping the brake with the nipple on or off.

After some further investigation, I removed the rear-left connection to the ABS line and fluid is getting that far. I tired an output test on the ABS and worked without issue. There is also no fluid coming from the brake line when removed leaving me to believe that the line is blocked. How does one go about unblocking this. BTW, the rear right brake flushes as per normal.

Any help would be appreciated.

[mtroxel]

So you are saying that you remove the brake hose from the caliper, someone steps on the brake, and no fluid comes out? And if so, what if you start working back towards the master cylinder, removing parts till you see fluid coming out. And if its the left rear, you might also confirm the right front brake is getting fluid also.

[Brillo]

What you wrote is not particularly clear ( I don't speak Australian - hah) so I'll make a few assumptions as follows: Fluid comes out of the ABS unit where the left rear line attaches, so there is no blockage between the MC and LR ABS output port. Also, no fluid comes out of the LR line where it attaches to the caliper. This suggests a blockage in the line between the caliper and the ABS unit.

If this is correct I would suggest blowing high pressure air through the brake line at the caliper end towards the ABS unit, brake line not attached to the ABS unit of course. Don't worry about using high pressure because by US and European standards brake systems (all brake lines and connections) are required to be able to withstand static pressures of 3000 psi. Wrap a white cloth and a large plastic bag around the brake line at the ABS unit so you don't blow brake fluid all over your engine compartment (brake fluid can be nasty stuff so take reasonable precautions) and also to capture the substance causing the blockage so you can examine it. If this doesn't work you might try to catheterize the brake line, again starting from the rear and working forward. Use something stiff like nylon weed wacker line with a nicely round the leading tip of the line. Don't force it if you meet resistance but try to keep rotating the nylon line as you push it through the brake line. I'm not sure how far these two ideas will get you but it's a start.

Depending on what is causing the blockage, such as rubber particles or corrosion products, further investigation may be warranted regarding the health of the whole brake system.

[Okedokey]

Thanks guys. Yes Brillo, you understood correctly. I am assuming Ill need a compressor to achieve the first suggestion. The second suggestions is clever, ill consider that too. Pain in the ass I say :)

MT, is the front right and rear left somehow connected? Is there a proportionating valve of some description that I may need to investigate?

[Brillo]

Definite PITA for sure, but you really want your brake system to be functioning properly - we can all agree on that. It's been a while since I dug into the A4 brake system. IIRC there is nothing after the ABS unit such as a proportioning valve, which you would find in older vehicles. I believe that pressure proportioning and anti-locking all happens at the ABS unit in the engine bay. I may be wrong so others please chime in on this.

[Okedokey]

Thanks mate.

[Okedokey]

OK for an update. I purchased a compressor and a vacuum pump. Was able to determine the following:

Brake line clear between nipple and ABS unit for the problem wheel.

However, fluid still doesn't pump to the left rear wheel.
Unable to vacuum or pressurize line from ABS to reservoir that feeds the left rear wheel.

Is that normal? How can I clear or fault find the issue. As the rear right wheel bleeds as per normal, I am assuming that the ABS, reservior and other components are fine, however the line that feeds from there on is blocked.

Any further ideas? BUMP!

[Okedokey]

Further bump, worked out the master lines aren't blocked after-all, must be the master cylinder, I have one incoming... bloody hell.