Brake servo failed?

Post by **Rob_Jennings** » Mon Dec 11, 2006 10:03 am

If the brake servo fails then I assume you are left with unassisted brakes?

I ask this because my brake pedal has become very heavy, although it still works. Also I thought a reasonable test of a servo was to stop the engine and pump the brake a few times, after a few tries the pedal should stiffen as the vacuum is exhausted? Trying this I get no such change in the pedal resistance.

This has happend after extensive work which required the car to be rolled over, I wonder if any oil not fully drained from the engine may have run back up the tube via the inlet manifold? When I get time and its dry I was going to remove the tubing and check I get a vacuum with the engine running. Any other things I can check for before assuming the servo had given up?

Cheers

Post by **bmcecosse** » Mon Dec 11, 2006 10:28 am

If fluid ran into the servo - then ideed it has probably failed. Easy check - remove the vacuum hose and plug it at the manifold - now you definitely don't have a servo, are the brakes any different ? Small car like a Minor doesn't need a servo anyway - I would just throw it away!

Post by **Rob_Jennings** » Mon Dec 11, 2006 10:35 am

You are quite right it does not 'need' it, but I am used to the modern box and swapping between the two can be fun. Also given thats its part of my insurance (policy is lower with it in place) I ought to think about repair or replacement.

I'll add running without the hose attached to the check list.

Post by **Rob_Jennings** » Mon Dec 11, 2006 11:51 am

looking at how a servo works....

The descriptions I can find show how the pedal links through the servo to the master cylinder and causes the inlet valve on the servo to open to use the atmospheric pressure and stored vacuum to amplify the force on the connecting rod to the master cylinder. And that removing your foot closes the inlet valve thus allowing the system to return off. It also makes sense that without a vacuum then your brake pedal force is the only thing acting on the master cylinder. Of course this is true for a modern all in one unit, but how does it work on the minor?

On the minor there is no linkage back to the master cylinder from the servo? so does that mean that the pressure in the brake system is used to open the inlet valve and this drives the output? but that would seem wrong, since as soon as it was high enough to open it would just run away to full brakes and there would be no way to stop this? is there a second master cylinder in the servo? and does the old master and pedal simply drive a piston in the servo that would have been the pedal linkage?

Post by **Rob_Jennings** » Mon Dec 11, 2006 1:16 pm

OK so chatting at lunch over the table I have worked out the following....

The old pedal and master cylinder drive the brake line that then pushes a new slave and master cylinder combination in the servo, this is where the force is amplified by the servo (new slave opens inlet, new master pushed on by servo pressures). If they reach endstops there is a bypass valve between them so that the 'original' circuit runs as normal from that point. The servo can only assist to the end of the second piston travel. the system relies on a return springs for these pistons once the brake force is released to restore their positions

so

heavy brakes may indicate any of the following:
vacuum fault from engine
rubbish in the servo stopping vacuum buildup
failed valve preventing vacuum holding.
servo diaphram failed so no assistance.
inlet valve/slave piston failed in servo so no servo effect.
pistons in servo failed/stuck at one end due to failed return spring.

many of those faults would mean a new servo I guess, given its age its probably beyond economic repair?

is there bleed point for the servo, or should it always end up expelling air through the bypass valve when bleeding brakes as normal?

sorry if this thread is a ramble, I'm just thinking allowed to get things clear for me I guess, so corrections are welcome.

Post by **bmcecosse** » Mon Dec 11, 2006 1:26 pm

I think you have it well summarised there. There is indeed an internal balance created based on the incoming pressure from your foot - the servo acts as an amplifier. Be aware thta in some failure modes the servo can 'drink' the brake fluid - which can eventually lead to complete loss of brakes. This can be seen as puff of white smoke from the exhaust after each brake application. Mate of mine had this with his MGB - threw the servo away, and is now delighted with his brakes - much more feel and not unreasonably heavy - and he has Parkinsons and so is not particularly strong limbed.