[Alex_P]

I've decided to tackle my brake issues head on by no longer applying careful thought and logic (which has got me nowhere in the last year) and attacking the whole system and throwing money at the problem!

Today I removed the rear brake servo and, when I removed the "vacuum air valve" found a reasonable amount of fluid and gunge in the "servo side".

I think I remember reading that this circuit should be air (or minus air) only, so could the presence of fluid here be the source of the air in my rear brakes which seems to have no decernable leak?

I thought it might be the servo as I seem to get air in the system with no measurable fluid going from the reseviour, so thought this could be the only potential place air could get switched for fluid with no net impact on the systems volume....

If fluid in the vacuum air valve is bad, I'm looking forward to a shiny set of new gold servos!

Cheers,

Alex

 

[kandlbarrett]

Master cylinder seal is leaking and also damage servo diaphragm from your description.

 

[Alex_P]

Master cylinder is known to be good, so the servo sounds like it's the issue?

 

[kandlbarrett]

Surely there must be an issue with the master cylinder for the fluid to leak from the cylinder into the servo or does the vacuum cause a problem with the cylinder even if the cylinder seals are good?

 

[Alex_P]

This is the twin servo RHD set up, so the MC and servos are separate.

My issue is there is brake fluid in the "air side" of the servo so I think it is leaking from the 'fluid side" of the servo and letting air in.

I've been told not to bother trying to rebuild the servos, so once I'm sure I've got fluid where I shouldn't, I'll replace both servos.

 

[hmitd]

You definitely shouldn't see fluid in the servo. I changed the seals in mine, wasn't too difficult. 2 new servos sounds expensive!

 

[Alex_P]

Thanks, I haven't got round to opening the bellows, but will do that tomorrow to see if there is any fluid in there...

Where I did find fluid was in the air valve on the bottom (as the servo is fitted), which seems wrong. The front one seems to be leaking there too, so I'll either rebuild or replace both, which isn't that much different in price!

 

[hmitd]

Mine leaked around the seal for the piston that triggers the air valve.

 

[hmitd]

By the way, be careful of the spring when you take the halfs apart.

 

[Alex_P]

So you had fluid up against the diaphragm which triggers the valve? If so, that seems to be exactly what mine is doing!

Cheers

 

[hmitd]

Yep, eventually got sucked into the engine with impressive white smoke.

 

[Alex_P]

I'd ruled the servos out originally due to the lack of white smoke, but then when I realised how little fluid was being lost, I though it was worth a closer look.

 

[kandlbarrett]

OK so I have used the wrong term. But the hydraulic actuator must have a leaking seal for hydraulic oil to be in the servo.

The smallest leak will affect pedal feel and braking effect.

 

[hmitd]

OK so I have used the wrong term. But the hydraulic actuator must have a leaking seal for hydraulic oil to be in the servo.

It's complicated in a RHD spider, effectively there are 3 master cylinders. The one at the brake pedal and two more in the 2 remote servos.

 

[kandlbarrett]

Yes, I realise that it isn't as simple as a standard system but theory is the same; if there is brake fluid in the servo that fluid must have come from the brake system so there must be a seal leaking.

 

[Alex_P]

We're all saying the same thing I think, and are in agreement that the "mini master cylinder" in the servo is leaking to get fluid where it shouldn't be!

I'm going to strip the rest of the servo to see if any fluid made it into the main bellows assembly, then decide if I'm going to rebuild or replace...

 

[Alex_P]

Opened the servo up last night and found about 1/2 a cup of fresh brake fluid in there!

This servo actually looks better than the other one, so both are getting changed.

I'm hoping that this is the root cause of my issue and, once it is fixed, I'll never have to bleed the sodding brakes again...

 

[kandlbarrett]

I am confused. If the servos are filling with brake fluid don't you also need to attend to the hydraulic system. There must be a failing seal or does the hydraulic actuator come with the servo?

 

[Alex_P]

The master cylinder pushes fluid out to the two servos when the brake pedal is pressed, these have a vacuum line to them from the inlet manifold which runs through the "vacuum air valve" when the brakes are not applied, keeping the bellows at equal pressure on both sides.

When the pressure from the master cylinder pushes on the cylinder in the servo it actuates the vacuum air valve, applying the vacuum to one side of the bellows. This collapse in pressure sucks the cylinder in the servo forward boosting the output hydraulic pressure which goes to the callipers.

What seems to happen is the seals on the servo cylinder leak allowing fluid into the vacuum air valve which can then go into the bellows and, eventually into the inlet manifold, which gives the characteristic white smoke. As part of this, while the leak is minor, air is swapped for the same volume of fluid in the circuit so you get spongy brakes without losing fluid from the reservoir and without finding it on the garage floor, hence the "mystery" of air getting in the system with seemingly no fluid loss.

Luckily the failing servo still gives you "unassisted" breaking and,on the back circuit, the loss in power isn't all that noticeable.