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Brake pedal sinking when engine on.

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Old Mar 4th, 2019, 11:18   #1
Karl88volvo
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Default Brake pedal sinking when engine on.

2005 v50 2.0d
Changed discs and pads all round, can get a good solid pedal when bleeding each corner, but as soon as i turn the engine on it fades away and pedal will slowly sink to the floor when held.

Its not leaking anywhere.

Can i bleed with engine on or will i damage anything?

Any suggestions?
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Old Mar 4th, 2019, 13:44   #2
StatusRed
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Vaccum?

I believe it's normal for you to feel the pedal sink a bit if you turn the engine on and have your foot on the brake as you do it.
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Old Mar 4th, 2019, 13:57   #3
Karl88volvo
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Vaccum?

I believe it's normal for you to feel the pedal sink a bit if you turn the engine on and have your foot on the brake as you do it.
Pedal sinks slowly all the way floor, braking is generally not very good, Bosch discs & EBC ultimax pads.
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Old Mar 4th, 2019, 14:56   #4
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That doesn't sound good. My old T5 had a slight fall in the pedal if you held your foot on it for a while at a standstill that was remedied by a combination of replacing the brake hoses (I went HEL braided) and fluid. It was cured after that but mine certainly didn't sink all the way towards the floor that's for sure.
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Old Mar 6th, 2019, 23:37   #5
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Had this issue after caliper rebuild.
Performed bleeding, fluid seemed ok but on a test drive brake pedal seemed softer. No sinking with engine off, but sinked to the floor in like 30 seconds with engine on without very much force on the pedal.
Repeating the bleeding procedure solved it, fluid went out without air for first couple of pumps, but then pushed out some bubbles. After that everything went back to normal. Pedal will still sink to the end but extremely slowly (will take minutes) and with a lot of force on the pedal, but as far as I know it does that in most cars.
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Old Mar 7th, 2019, 00:06   #6
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Diesels do not create a vacuum by suction like a petrol engine does , instead they have a mechanical vacuum pump . A petrol can only generate a limited vacuum & is dependant on the butterfly in the manifold creating negative pressure when running .

The diesels mechanical pump continues to generate lower pressures continually & this can lead to the brake pedal sinking .

I encounter this issue nearly everytime I MOT a diesel vehicle & some are worse for the problem you have encountered , it is easily diagnosed , you switch off the engine , press the pedal until the pedal become difficult to press . Keep your foot on the brake pedal , start the car & feel the pedal sink as vacuum is generated . Leave your foot on the pedal & it will continue descend as vacuum increases
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Old Mar 7th, 2019, 07:50   #7
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I had this problem on my 2.0d
In my case it was the clutch slave cylinder leaking in the bell housing (difficult to see as not leaving any fluid on the ground as inside bell housing)
The brake pedal would need pumping a few times to get pressure, and led to clutch slip so ended up having clutch, flywheel and slave cylinder replaced.
Keep an eye on dropping brake/clutch fluid level.

I believe slave cylinder may be a recall on some models?

Last edited by rowdy; Mar 7th, 2019 at 09:20.
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Old May 5th, 2019, 11:02   #8
minimunky
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I have the exact same problem and have even changed the master cylinder in an attempt to resolve the problem!

Am I right in thinking that I should consider changing the vacuum pump now? Just checked and it is creating a vacuum - no idea if is is sufficient.

What I'm trying to understand is why has a change in pads and bleeding of the system highlighted this potential issue?
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Old May 17th, 2019, 16:18   #9
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Quote:
Originally Posted by minimunky View Post
I have the exact same problem and have even changed the master cylinder in an attempt to resolve the problem!

Am I right in thinking that I should consider changing the vacuum pump now? Just checked and it is creating a vacuum - no idea if is is sufficient.

What I'm trying to understand is why has a change in pads and bleeding of the system highlighted this potential issue?
Hi
late reply, but just checking - are you sure that pedal sinking is actually an issue and you are not mistaking it for power brake-assist ? When I stomp on the pedal full force and then lower the stomp force just by tiny bit, I can feel pedal wants to go to floor but its due to the emergency brake assist (it will apply full braking force assist when emergency braking is detected). But otherwise, no, once you press brake it should hold and you should feel proper pressure.
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Old May 21st, 2019, 23:26   #10
Karl88volvo
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Quote:
Originally Posted by LizardOfBodom View Post
Hi
late reply, but just checking - are you sure that pedal sinking is actually an issue and you are not mistaking it for power brake-assist ? When I stomp on the pedal full force and then lower the stomp force just by tiny bit, I can feel pedal wants to go to floor but its due to the emergency brake assist (it will apply full braking force assist when emergency braking is detected). But otherwise, no, once you press brake it should hold and you should feel proper pressure.
Hi, engine off - pump peddle - goes solid but if held it will sink away,
Engine on - worse.

No loss of fluid, passed Mot but just dont feel as good as they shud imo.
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