v50 brake booster issue - pedal to floor?

[alexx]

Hi all, I've been messing around with my 08 2.0d v50 - the brake pedal went to the floor on a long trip and I got recovered. I suspected it was the abs module after some googling.. however that didn't improve anything.. next I got my hands on a master cylinder... still the same issue.. so now I'm thinking its the brake booster.

If I remove the vacuum pipe from the booster when the engine is running the pedal stays solid and doesnt sink to the floor when holding under pressure.

If the vacuum is connected and the engine running it sinks to the floor under pressure... is that good enough evidence that im now on the right tracks?

i used a vacuum and also a pressure bleeder so im confident theres no air in the system either.

thanks

[IainG]

Hi Alexx
My thought would be, if the pedal is going to the floor, where is the fluid from the master cylinder going to?
It is a sealed system so as the pedal goes down either the pistons on the calipers are moving, there is air in the system and is being compressed, fluid leaking out of the system or fluid is leaking past the master cylinder seals, back to the reservoir. The booster is just adding extra pressure to assist your foot pressure.

I can’t see how a servo/ booster could cause this issue. That is just my reasoning/logic and I might be wrong??

[alexx]

Hi Iain, thanks for your thoughts - I had this thought too.. but the fluid is not leaking out of the system anywhere so if there is a leak its internal.

However when the system is pressurised (with a bleeder cap) it holds 15psi

And when the car is turned off the pedal holds pressure perfectly. Its only when the car is running that the vacuum seems to be causing an issue to the brake booster.

Can the brake booster not fail causing it to lose pressure its putting on the master cylinder whilst under vacuum?

Its either that or its still a dodgy abs sensor, however the one I replaced and then removed caused an abs fault to show on my dash (even when using my electrics module) so I just put the old one back in.

[Clan]

Originally Posted by IainG

Hi Alexx
My thought would be, if the pedal is going to the floor, where is the fluid from the master cylinder going to?
It is a sealed system so as the pedal goes down either the pistons on the calipers are moving, there is air in the system and is being compressed, fluid leaking out of the system or fluid is leaking past the master cylinder seals, back to the reservoir. The booster is just adding extra pressure to assist your foot pressure.

I can’t see how a servo/ booster could cause this issue. That is just my reasoning/logic and I might be wrong??

That fluid is going somewhere, either into an air bubble somewhere.. Have you properly bled the hydraulic stem ?

OR the only other spongy thing could be one or more rubber brake hoses expanding due to age? However I have never come across this on a volvo of any age especially one as "new " as yours.

[alexx]

properly bled 4x, all the way through each line with fresh fluid under vacuum and again under pressure bleeding to make sure.

no air bubbles running for a good 0.5 litre per line then closing the nipple.

the abs circuit could still be at fault leaking internally... or i've bought another faulty master cylinder (fairly unlikely) or the brake booster could be at fault.

I just dont know why the pressure on the pedal is only creeping when the car is turned on ie under vacuum.

I know a brake booster usually fails making the pedal hard but can it not fail by not transferring the pressure onto the master cylinder and compressing itself?

[IainG]

Hi

.I know a brake booster usually fails making the pedal hard but can it not fail by not transferring the pressure onto the master cylinder and compressing itself?

With a booster or servo.
It is fitted between the foot pedal and the master cylinder. It provides a solid link between the 2.
It is basically a solid rod linking the pedal and master cylinder with a diaphragm attached to the shaft inside a housing. (Which gives assistance using vacuum from the engine)
It can’t compress on its own and is designed so that if the servo fails there is still a solid link between the pedal and master cylinder. Although without any assistance. Hence having to press the pedal harder when there is no servo assistance

[alexx]

gotcha! thanks for the explanation

so it's got to be either:
1, air
2, dodgy abs block
3, another dodgy master cylinder

probably in that order of likelyhood.

I'll do another bleed just to be sure before trying another abs block.

and I'll take that as an opportunity to look around the flexi pipes for any distortion under pressing the pedal with the engine running to see if that shows up anything sinister

Thank you

[IainG]

Hi
Good luck