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Customer reply replied 14 days ago
Audi A4 2007 Quattro with 2.0 Turbo...Just did brake service on all 4 wheels, replaced the pads only. Did not bleed the brakes or open any brake lines.The break pedal will not hold pressure now and there's no stopping power. I pump the brake pedal and it gets stiffer after several pumps but once I keep pressure on the pedal (after it gets stiff) it goes all the way to the floor.Inspecting the car there seems to be a small spill of fluid near the passenger front, more than a foot from the tire but still under the engine compartment. It's less than an ounce of fluid and turning on the car and pumping the brakes doesn't seem to add any more fluid to the spill. I can see brake fluid on the top of the brake fluid reservoir, it looks like fluid has leaked through the cap and onto the top of the reservoir, then down into the engine compartment around the master cylinder.Your help is appreciated.
Answered in 19 minutes by:
5/6/2018
Matt
Matt, Engineer
Category: Audi
Satisfied Customers: 23,069
Experience: Mechanical Engineer with 20 years experience in the auto industry, 8 yrs in formula 1 engine testing
Verified

Hello and welcome to Just Answer

I'd agree that your fluid leak just sounds like where you have pushed the wheel caliper pistons back which has forced fluid back up the system and out the breather hole in the master reservoir cap

however what can happen if you have a worn or torn caliper seal is that you can introduce air into the system when you push the piston back

So I'd suggest the next step is to bleed the brakes to see if this restores the pedal feel

I favour having a clear hose from the nipple to a jar 1/2 full of old fluid and with the nipple closed your assistant pushes the pedal up and down a few times then holds it down then you crack open the nipple and close it again whilst fluid is still flowing.

Repeat this a few times on all calipers and start with the caliper furthest away from the master cylinder.

If you still have an issue then it could be a flexible brake line is ballooning under pressure, this can't always be seen from the outside so to test this use a hose clamp on each line in turn to close off then lines one at time . Push the pedal in between and see if you can feel any difference as each line is closed off.

if this doesn't help then its possible that you need to run the ABS pump to move any trapped air inside
the dealer method is to use the diagnostic tool to cycle the ABS during bleeding

however I've had good results buy just taking the car for short drive and stamping on the pedal at low speed to lock the brakes and trigger the ABS and then re-bleed

if any of the caliper seals are torn they will need to be replaced and you can rebuild a caliper yourself providing you have a foot pump or a compressed air supply

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Matt
Matt, Engineer
Category: Audi
Satisfied Customers: 23,069
Experience: Mechanical Engineer with 20 years experience in the auto industry, 8 yrs in formula 1 engine testing
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Customer reply replied 14 days ago
Hello Matt,Thank you for the quick and concise response, a few follow up questions:1. Are there there any symptoms or specific diagnostic steps I can take to see if a caliper/caliper seal is bad? This would also be useful to narrow it down to the bad wheel.
2. In your experience, what are the chances this is a bad master cylinder/ master cylinder seal? Are there diagnostic steps to check that specifically?

Hello

when you replaced the brake pads did you notice if any of the round circular seals around each piston were torn or distorted at all?

as this would usually mean thats the likely suspect

the outer seal is a dust /water seal and doesn't do any hydraulic sealing but its protected by the outer seal

if the master seal is gone then usually you will have a a greyish black fluid in the master cylinder as the tiny bits of rubber from the master seal break off and discolour the fluid

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Customer reply replied 14 days ago
Hello Matt,Just bled the brakes and the car is derivable now and the situation is greatly improved, but when you're sitting for any length of time the brake pedal slowly loses pressure and goes to the floor. Maybe I just need to bleed the brakes more?When I compressed the pistons on the back brake pads I noticed the circular seals where warping as I spun the piston to get it back into it's starting position. I assumed this was normal since they have to rotate to be compressed. I guess you're saying the seals on the back calipers are probably bad?I haven't noticed any greyish/black fluid from the bleed so hopefully that's a good sign.

Hello

this sounds like there's still some air in the system and possibly inside the ABS unit

so I'd say to try the hard stop method I mentioned in the 1st reply

if the dust seals were warping / crinkling up though as you turn the pistons then this indicates they are past their best

however if they've not actually torn or split then the main seal should be OK

when it comes to bleeding then it may be worth investing in a vacuum bleed tool like this

https://www.amazon.co.uk/Mityvac-MITMV8500-Silverline-Vacuum-Pressure/dp/B0002SQYUA/ref=sr_1_1?ie=UTF8&qid=1486586815&sr=8-1&keywords=mityvac+mv8500&gclid=EAIaIQobChMI0qr0__7x2gIVo7vtCh2_jgrIEAAYAiAAEgLxBPD_BwE

as I've found they are very effective at getting the last bit of air out of system

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Customer reply replied 13 days ago
Perfect, just need to learn how to bleed brakes properly or get a tool then. :)Thank you for your help Matt, much appreciated.
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Matt, Engineer
Category: Audi
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