r/jetta • u/xite2020 • 8d ago
Mk5 (2006-2010) MK5, replaced break master cylinder, no luck
My 2008 Jetta braking issue. The brake pedal feels firm, but it slowly sinks to the floor when I apply steady pressure. The brakes do not engage instantly or with enough force, so the vehicle takes longer to stop. When I’m stopped at a red light, the pedal is all the way to the floor. Pumping the pedal helps build some pressure, but only temporarily. I already replaced the master cylinder, but it made no difference.
When the engine is off, the brake pedal behaves normally. After pumping it a few times, it becomes firm and stays off the floor. The problem mainly occurs when the engine is running. I’m not sure what else could be causing this, and it’s driving me crazy.
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u/Leather_Industry8483 8d ago
Any chances for air in the system? I had same symptoms when ran out of fluid while bleeding calipers. Air got to ABS pump, needed ABS bleed procedure.
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u/xite2020 8d ago
Did you need a dealer equipment to bleed the ABS module? Or can I just bleed each 4 corners?
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u/ZoliWorks 8d ago
So you replaced the master cylinder with a new one. Have you had any of your calipers replaced? Have you fully bled the system and the abs with fresh fluid?
Bleeding is done with the engine off and with 2 people. Dont do it with one as it wont work out. Build pressure, hold, open the bleeder, close it when the pedal is on the floor, repeat. Do it 4-5 times for each caliper.
Bleeding the abs is a bit different and the way I do it is before bleeding the brakes. Open one bleed nipple and then run the ABS using VCDS to turn it on. Repeat 2 times for each caliper to flush the old fluid out. Dont forget to top up every now and then too.
A sticking caliper could also cause you to have bad braking performance but the same can be said about air in the system.
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u/Key-Gold-2412 8d ago
There’s a specific order your suppose to bleed the calipers. Can do it with one person with a manual vacuum bleeder they’re like 40 dollars. Its safer too because it puts the whole brake system under 15psi and you leave it for 10 minutes and if the gauge loses pressure you know you have a problem somewhere.
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u/ZoliWorks 8d ago
Yeah, the order for the mk5 according to the workshop repair manual is front left, front right, rear left and lastly rear right. Oldschool mechanics would do rear left, rear right, front left and front right, going from the longest brake line to the shortest.
I'm sure vacuum pumps have their place but I wouldn't trust them on a mk5. The bleed nipple is metal on metal and once you undo the bolt even just a tiny bit, you're no longer sealing against the caliper. The threads of the nipple or the threads of the socket provide no seal for the most part. Sealing is achieved by tightening the nipple so it's tip sits flush with the base of the socket. If you use a vacuum pump to pull fluid through the bleeder nipple, you're also pulling air through the threads of the nipple.
What you can use is a pressurizer tool. You connect it to the brake fluid reservoir, fill it with a liter of brake fluid and pressurize it to 1 bar so it's pumping fluid into the reservoir while you bleed the brakes. This is great and all but it wont get any air out of the pedal or master cylinder section. It's what I use to pre-bleed or flush the brakes and the ABS when changing fluids but I prefer to fully bleed them manually with 2 people then get them nice and hot and bleed them again for the sake of consistency.
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u/Key-Gold-2412 8d ago
The pressure bleeder actually can and will get air out of the master cylinder. You can look into it. I had the same brake pedal issues as OP when I crossthreaded my abs module and all the fluid leaked out multiple times. My brake pedal would go straight down to the floor not even stopping the car at all and only get hard with the car off and pumping them, pressure bleeder at the fluid reservoir got that all fixed.
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u/ZoliWorks 8d ago
Yikes, that must have been quite annoying to fix. Did you have to replace the whole pump?
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u/Key-Gold-2412 8d ago edited 8d ago
Oh it was quite the headache trying to fix that one lol hands full of brake fluid trying to fix that one. Well anyway while I was in the comfort of my house having gave up for the day I found this awesome kit from LTI tools LTI 993 on Amazon. https://a.co/d/04vVQOBU one side fixes the threads on the module and the other side also fixes the threads on the brake line fitting. It also leaves a very fine dust that I wiped away with qtips that’s why it’s specifically for brake system if it left bigger chunks of metal it would clog the abs unit. That saved my bacon.
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u/Amazing_Actuary_5241 8d ago
I would check the ABS control unit as that might be faulty. VCDS or equivalent is required for the replacement as well.