Brake servo help

[Ministrone]

My Dolly has developed an issue while being parked up for the last week.
With the engine running, when the brake pedal is pushed and held down, there is a continuous hiss from the servo and the engine revs rise by around 3 or 400rpm.

The brakes still feel good and the servo seems to be working.
Does this point to the non-return valve being the problem?

[xvivalve]

It would be my first point of investigation...

[Ministrone]

I’ve been out and swapped the valve for an old spare and it still sounds like air rising in from inside the car. I guess i need to swap the servo.

[xvivalve]

Before you do that, take the non return valves out and see if you can blow and suck air through them? If you can, they're rooted; you should be only able to do one or the other depending which way round you have it...

[Ministrone]

Yep!
Both checked, both fine.
I have a spare servo but it’s not pretty, has anyone reconditioning a servo using a seal kit?

[Bumpa]

I assume we are talking about the in-line servo, mounted between the pedal and the master cylinder. I have the same unit on my MGB and I used a seal kit on it more than ten years ago. No trouble from it so far. Is that tempting fate?

[xvivalve]

Then it all depends where you can hear the hiss:

If it is inside the car, the canister normally exhausts into the car via a filter that is inside the nose cone where the pushrod comes out. A hiss is normal as the canister purges when you apply the brakes, but if it is continuous there is a seal here that needs replacing. Unfortunately that involves removal of the servo and opening up the canister, which depending on the condition of the canister can be no mean feat (as can sealing the canister again afterwards)

If the hiss is louder in the engine bay, then it is a simpler operation as it is leaking around the pushrod which operates the master cylinder; remove the master cylinder and the seal is accessible without either removing or disassembling the servo.

Beware, some servo seal kits are not fully comprehensive.

[xvivalve]

Oh, and on an early car with single line hydraulics DO NOT be tempted to test your servo whilst there is no fluid in the master cylinder as this will write off your servo.