Sinking Brake Pedal

geordie john

geordie john

Joined
Nov 25, 2023
Posts
54
Likes collected
30
Location
Scunthorpe, UK
Funster No
100,044
MH
Renault T821
Exp
2019
I have a Delphin T821 Tag with a sinking brake pedal with engine running. Mot is Due middle of Dec. Need urgent help as am running out of options. Started with handbrake being poor and discs pitted changed discs and pads rear adjusted handbrake all started from there. Peddle just sinks so i changed master cylinder bled them and bled them and pressure bled them still the same just cant get peddle to stay firm when engines running. There are no leaks as it maintains pressure with tester and i have visibly looked. Anything else i have missed. thanks in advance There's a drink in it for anyone who can tell me how to solve it thanks again.
 
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Spriddler said:
If it's not leaking brake fluid it sounds to me like it could be a brake servo/vacuum issue.
Click to expand...
that's my next step i have a servo unit on order should be here Monday. My brake peddle is firm when the engines not running that's what's thrown me but will give it ago thanks otherwise its ABS pump or a bad airlock.
 
Upvote 0
If brake fluid hasn't been changed at service intervals quite likely to be abs unit . Unlikely to be servo if pedal slowly dropping as servo leak leads to hard pedal . Servo pump down test , stop engine , pump pedal till hard . then start engine pedal should move towards floor.
 
Upvote 1
Have you got access to get a computer hooked up and run an abs force.

If there is air trapped in the unit it may move it or you can crack each union whilst gently pressing the pedal as if you are bleeding the brakes at the callipers.

As said it may just be the abs unit.

Good luck 🤞
 
Upvote 0
geordie john said:
that's my next step i have a servo unit on order should be here Monday. My brake peddle is firm when the engines not running that's what's thrown me but will give it ago thanks otherwise its ABS pump or a bad airlock.
Click to expand...
Definitely a servo leaking problem, I had same problem, replaced the brake servo and all was good after that.
 
Upvote 0
wiff2110 said:
Definitely a servo leaking problem, I had same problem, replaced the brake servo and all was good after that.
Click to expand...
I also reckon it could be a servo problem as I suggested in my post #2
I traced it to an almost invisible tiny split in the rubber vacuum hose right where it connected to the the servo unit. I just cut 2" off the split end and reconnected it.
 
Upvote 0
tonyidle said:
It is normal for the pedal to be very firm once the servo vacuum has been cleared (engine stopped). It will be much softer with the engine running but shouldn't creep after initial application.
Click to expand...
Correct.

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Upvote 0
tacr2man said:
If brake fluid hasn't been changed at service intervals quite likely to be abs unit . Unlikely to be servo if pedal slowly dropping as servo leak leads to hard pedal . Servo pump down test , stop engine , pump pedal till hard . then start engine pedal should move towards floor.
Click to expand...
Yeah it does that it goes right to the floor lol
 
Upvote 0
Silver-Fox said:
Have you got access to get a computer hooked up and run an abs force.

If there is air trapped in the unit it may move it or you can crack each union whilst gently pressing the pedal as if you are bleeding the brakes at the callipers.

As said it may just be the abs unit.

Good luck 🤞
Click to expand...
My theory probably still air in there. But getting it out is another matter without the tool.
 
Upvote 0
Spriddler said:
I also reckon it could be a servo problem as I suggested in my post #2
I traced it to an almost invisible tiny split in the rubber vacuum hose right where it connected to the the servo unit. I just cut 2" off the split end and reconnected it.
Click to expand...
will be checking that once my servo is here thanks
 
Upvote 0
geordie john said:
I have a Delphin T821 Tag with a sinking brake pedal with engine running. Mot is Due middle of Dec. Need urgent help as am running out of options. Started with handbrake being poor and discs pitted changed discs and pads rear adjusted handbrake all started from there. Peddle just sinks so i changed master cylinder bled them and bled them and pressure bled them still the same just cant get peddle to stay firm when engines running. There are no leaks as it maintains pressure with tester and i have visibly looked. Anything else i have missed. thanks in advance There's a drink in it for anyone who can tell me how to solve it thanks again.
Click to expand...
geordie john said:
I have a Delphin T821 Tag with a sinking brake pedal with engine running. Mot is Due middle of Dec. Need urgent help as am running out of options. Started with handbrake being poor and discs pitted changed discs and pads rear adjusted handbrake all started from there. Peddle just sinks so i changed master cylinder bled them and bled them and pressure bled them still the same just cant get peddle to stay firm when engines running. There are no leaks as it maintains pressure with tester and i have visibly looked. Anything else i have missed. thanks in advance There's a drink in it for anyone who can tell me how to solve it thanks again.
Click to expand...
 
Upvote 0
geordie john said:
that's my next step i have a servo unit on order should be here Monday. My brake peddle is firm when the engines not running that's what's thrown me but will give it ago thanks otherwise its ABS pump or a bad airlock.
Click to expand...
That proves it’s your servo
 
Upvote 0
geordie john said:
I have a Delphin T821 Tag with a sinking brake pedal with engine running. Mot is Due middle of Dec. Need urgent help as am running out of options. Started with handbrake being poor and discs pitted changed discs and pads rear adjusted handbrake all started from there. Peddle just sinks so i changed master cylinder bled them and bled them and pressure bled them still the same just cant get peddle to stay firm when engines running. There are no leaks as it maintains pressure with tester and i have visibly looked. Anything else i have missed. thanks in advance There's a drink in it for anyone who can tell me how to solve it thanks again.
Click to expand...
Yes defo try a servo on it, not that much off Euro Car Parts or ebay, if pedal was sinking on it's own, or sticking down would said concentric slave cylinder 👌
 
Upvote 0
Sammy 🍺 said:
Yes defo try a servo on it, not that much off Euro Car Parts or ebay, if pedal was sinking on it's own, or sticking down would said concentric slave cylinder 👌
Click to expand...
Sorry numpty, that's clutch not brakes lol 🙈

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Upvote 0
geordie john said:
I have a Delphin T821 Tag with a sinking brake pedal with engine running. Mot is Due middle of Dec. Need urgent help as am running out of options. Started with handbrake being poor and discs pitted changed discs and pads rear adjusted handbrake all started from there. Peddle just sinks so i changed master cylinder bled them and bled them and pressure bled them still the same just cant get peddle to stay firm when engines running. There are no leaks as it maintains pressure with tester and i have visibly looked. Anything else i have missed. thanks in advance There's a drink in it for anyone who can tell me how to solve it thanks again.
Click to expand...
I have a Fiat Ducato Tag Axle Bus which had the same issue a few years ago. I changed the same components but the perceived 'problem' was still there.
It was in the end as said on this forum, 'diesel creep'.
Brakes work fine and it passed the MOT a few weeks ago.
I think the soft feel of the brake pedal can be misleading and attempts to prompt the 'creep' can make you think there is a problem.
 
Upvote 0
geordie john said:
I have a Delphin T821 Tag with a sinking brake pedal with engine running. Mot is Due middle of Dec. Need urgent help as am running out of options. Started with handbrake being poor and discs pitted changed discs and pads rear adjusted handbrake all started from there. Peddle just sinks so i changed master cylinder bled them and bled them and pressure bled them still the same just cant get peddle to stay firm when engines running. There are no leaks as it maintains pressure with tester and i have visibly looked. Anything else i have missed. thanks in advance There's a drink in it for anyone who can tell me how to solve it thanks again.
Click to expand...

Colin57 said:
I have a Fiat Ducato Tag Axle Bus which had the same issue a few years ago. I changed the same components but the perceived 'problem' was still there.
It was in the end as said on this forum, 'diesel creep'.
Brakes work fine and it passed the MOT a few weeks ago.
I think the soft feel of the brake pedal can be misleading and attempts to prompt the 'creep' can make you think there is a problem.
Click to expand...
Thanks for the reply Theres diesel creep and theres peddle drop mine goes down quite easily
 
Upvote 0
Colin57 said:
I have a Fiat Ducato Tag Axle Bus which had the same issue a few years ago. I changed the same components but the perceived 'problem' was still there.
It was in the end as said on this forum, 'diesel creep'.
Brakes work fine and it passed the MOT a few weeks ago.
I think the soft feel of the brake pedal can be misleading and attempts to prompt the 'creep' can make you think there is a problem.
Click to expand...
This is probably caused by the distance between the 4 x corners (long way) as long as you don't get brake fade, which is normally either air lock bubbles, or vac leak from servo or rubber pipes, I'm sure my pedal on my new 2023 Fiat Ducato feels a bit spongey on braking, but stops perfectly well !
So I'm inclined to agree with you Colin.
You will never get a pedal feels like on a car or 4x4 if you think about it 🤷‍♂️
 
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