How should a brake servo operate?

[ArgyllMC]

Basic question which has had me scratching my head over the weekend.
Defender 300 tdi failed MOT last week due to brake servo failure. Did a couple of tests and confirmed Vac pump pulling air ok. Manually testing servo revealed leak round bolt, guessing failed internal seal.
New servo ordered and fitted, system bled and no improvement. Having now been through the system I have found that the servo only holds a Vacuum in the pedal fully up position, slightly depressing the pedal causes vacuum to be lost and a sound of air escaping, in this case the air I suppose is rushing in though. My question is whither this is normal or not? Looking at the construction I can't see how the internal diaphragm would ever seal when the pedal is anything other than up or very slightly depressed. However I can see the logic in the vacuum being broken to let the pedal etc return to the rest/raised position.
Any thoughts would be great.

 

[Mursal]

Before you touch the pedal you have a vacuum both sides of the diaphragm. When you hit the pedal air is allowed to enter the pedal side of the diaphragm, giving a pressure difference of up to 1bar. Helping to apply enough force to get the pads to act on the disks. The low working pressure difference (-1bar = perfect vacuum) means the diaphragm has to have a large area for any meaningful force to be applied. Force = Pressure x Area
Older stuff had shoes and drum brakes that were self servo, as soon as the shoe made contact with the drum, it tended to bind on, until released.

Hope this helps

 

[ArgyllMC]

Thanks for that.
I am starting to think it may be a vac pump issue. I have plumbed up a pipe to let me try testing it be sucking. With pedal at rest I suck until I have a Vacuum and can hold the pressure with my tongue, servo holds vacuum unhindered. Depress pedal,vacuum breaks after a little movement, air enters servo chamber and vacuum is lost. With pedal depressed you can suck air freely through servo, there is no seal between one side of chamber and the other.
Although leaning toward a pump issue I am not convinced. Surely the assistance should be there through the full pedal travel?
Apologies if I am being thick here.

 

[mfarrow]

If the vacuum is not holding on the engine side when you depress the pedal, then you have a holed diaphragm, or faulty internal valve.

Is there a non-return valve external to the servo? Have you checked that it's functioning correctly?

The failure mode for MoT purposes I'm guessing is due to the servo not holding vacuum when the vacuum hose is disconnected, engine stopped/on overrun etc. If so it's good for the garage to spot this failure, I've had some which haven't.

 

[ArgyllMC]

Garage failed it for vacuum issue as with pedal pumped hard before starting after there is no noticeable assistance.
Having done the same with my primitive test pipe in this position air simply free flows through servo, vacuum can never be established. So is it therefore a faulty servo out the box, problem is looking at the design internally once off the rest position there could never be a seal between the two sides of the diaphragm....

 

[ArgyllMC]

Oh yeah one way valve operates correctly, just to rule that out!

 

[Mursal]

You will have a vacuum both sides until you let air into one chamber.

Then there must be a differential of pressure on the two sides of the diaphragm, or it would be pointless using it. There would be no assistance. So if you cant see the differential pressure compartments when you hit the pedal, there must be something missing.