Brake pedal sinking when engine on.

Karl88volvo · Mar 4th, 2019, 10:18

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?

StatusRed · Mar 4th, 2019, 12:44

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.

Karl88volvo · Mar 4th, 2019, 12:57

Quote:

Originally Posted by StatusRed

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.

iainmd · Mar 4th, 2019, 13:56

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.

hoks · Mar 6th, 2019, 22:37

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.

dingov70 · Mar 6th, 2019, 23:06

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

Mar 4th, 2019, 10:18	#1
Karl88volvo Junior Member Last Online: Jul 3rd, 2020 14:45 Join Date: Oct 2016 Location: Durham	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?

Mar 4th, 2019, 12:44	#2
StatusRed Premier Member Last Online: Yesterday 09:08 Join Date: Jul 2015 Location: Glasgow	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. __________________ 2009 S40 SE Lux 2.0i \| 2022 Suzuki V-Strom 650 \| 2002 Audi TT 225 Previous: C70 Coupe 2.4i \| Saab 9-5 Aero \| Daihatsu SporTrak

Mar 4th, 2019, 13:56	#4
iainmd Member Last Online: Yesterday 14:11 Join Date: Nov 2004 Location: Fife View Photos By: iainmd	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. __________________ Present: 2009 Volvo V50 1.6D DRIVe SE Past: 2004 Volvo S40 T5 SE

Mar 6th, 2019, 23:06	#6
dingov70 Master Member Last Online: Yesterday 06:42 Join Date: Jan 2015 Location: Blackpool	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 __________________ To stupidity & beyond

Mar 6th, 2019, 22:37	#5
hoks Junior Member Last Online: Oct 14th, 2019 13:54 Join Date: Feb 2018 Location: Riga	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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