hi everyone

I need to do is ask your advice about the brake servo.....

Just fitted new pads, disc, drums, shoes, wheel cylinders and bled the whole system out with an Ezibleed system and the engine off. Then to make sure I bled the fronts with the traditional method of having a friend depress the brake pedal and locking off the nipple when bubbles stop flowing. This was done wi the engine on (in case the servo needed to so anything).

Anyway the brakes are fully bedded in now and do stop the car but the pedal is travelling too far before there is sufficient bite. The car is not unsafe but just feels nasty to drive. I am now wondering if the brake servo is knackered?

From what I understand the servo is provides extra effort on the brakes so that the foot pedal does not need to be depressed so hard.

Is there a way to test the servo? Are they likely to malfunction?

Cheers

Russ

Update:

Just had a quick look. I traced the pipe from the servo to the manifold. In the middle of it there is what looks like a valve.

From looking in my Haynes I think they call this the 'check' valve which is a one way valve. I removed this pipe complete with valve from the car and fired it up. It ran like a dog with the engine accelerating then nearly stalling (presume this was because there was an air leak affecting the ecu timing etc) anyway the brake pedal felt much firmer but there were next to no brakes!! I only took it on the drive 2 yards and it took about another 2 yards to stop the car at about 3 mph with the pedal literally on the floor!!

So... when all the gear is connected the engine runs fine. This makes me want to rule out the possibility of an air leak in the servo unit?

The other conclusion I can make is the servo is doing something even if it feels like it isn't doing much at all. Maybe the problem is it works, but not quite as well as it should?

Whilst I had the pipe and vale off the car I blew down it and the valve works fine, you can only blow down it one way.

I then hooked everything back up to the car and took it for a spin. The first couple of minutes were ace, the brakes felt like they should, really sharp. I did however notice after a few minutes the pedal started to feel weak again (pedal nearly at floor before brakes start to do much) the same problem I had before although not quite as bad as before.

It certainly looks like either the servo or the valve is on its last legs and playing up a bit.

What do you think the next stage of testing should be? Is it more likely to be the servo or the valve?

I see lots of used servos for sale but are these likely to be dogs too?

Any advice appreciated please! All I want its the car to be nice to drive :( why does my Polo show me no love!?

Re-bleed it with the engine off

Edited 5 June, 201115 yr by mc_hawkings24

With the engine off, keep pumping the brake pedal until it gets hard and will not travel any further, then with your foot hard on the pedal, start the engine you should find the pedal gives a bit further travel, after about 2 mins switch off the engine, and then keep pumping the pedal and it should get harder and the pedal travel will be less on each push of the pedal, if this happens the, servo and check valve are working correctly, your symptoms sound more like the master cylinder is partially leaking passed the internal piston seal.

All brake bleeding should be done with the engine off, or if you have an electric pump for an ABS system, with the ignition off, this means that in the event of engine/vacuum/servo/electrical failure, the braking system will still have maximum efficiency, even if you have to have calf muscles like Popeye!! to achieve maximum braking

From what you say, your servo is working correctly and needs no further attention.

One possible cause of the excess pedal travel may be that you have air in the rear wheel cylinders. The problem is that the automatic adjusters hold the linings very close to the drums, this is good in normal operation, but when you are bleeding the brakes, the pistons move out and do not return, leaving a possible air space in the cylinder, as the air is compressible, the pedal finishes up with too much travel.

The answer is to reduce or even eliminate this air space, this can be done by lifting the auto adjuster wedge and allowing the brake spring to pull the shoes together, thus pushing the pistons back into the cylinder.

Access to the wedge can be gained through a wheel bolt hole.

You may have to lift the wedge more than once as bleeding progresses.

Hope that this is of some help.

Regards.

cheers guys.

Not sure I should ask this as i got abuse last time but...

Would it not work if i removed the drums and pushed the shoes together (to make sure wheel cylinders are home) whilst at same time move the wedge? If I bled the rear brakes whilst doing this, is this what you mean?

Thanks R

You can if you wish! You could even put a 'G' clamp across, or a luggage strap round the shoes once that they are fully retracted,this would hold the pistons in whilst you bled the brakes, but it is not normally necessary.

Regards.

cheers guys.

I think you're right, i can;t be @rsed taking the drums off again :( I'll try your trick of pushing the wedge up and then re bleeding the rear cylinders.

As an update, i tried something today. Where the air hose goes in to the servo is a rubber grommet, I pulled this out slightly and heard the air hiss out. I then replaced the hose and for the first few yards of driving the brakes were fantastic, felt exactly like they should; nice solid and firm with little pedal travel before the bite. However....after braking once or twice the sponginess came back!!

Does this give you lads any ideas as to what may be the problem? The brake fluid level doesn't go down so I doubt its loosing fluid, although could air be getting into the fluid pipes/hoses somewhere? Seems odd that when I drain the air out the servo and then drive its ace and 2 mins later it goes spongey again. I'm confuzzled.

R

This all sounds a bit upside down to me. First of all, when you heard the air hissing, it would be going INTO the servo, not out of it!

Are you sure that you are not mistaking the servo action itself for sponginess in the brakes? When you let the air into the servo and the brakes felt 'better', this could have been due to you providing all of the effort, with foot pressure alone (unless you have feet like an elephant) you will not get the same effect as when the servo is working with you.When the servo is helping you, it is quite possible for each brake hose to balloon slightly ( this is why people fit braided hoses), for the rest of us, we just live with it, but you will always have more pedal travel with a working servo than you will without it.

Apologies if the above is not the case with your car, in which case, the next thing that I would be looking at is to try and find out if someone has fitted a wrong master cylinder, the ones fitted to none servo equipped vehicles tend to be of a SLIGHTLY smaller bore than to those vehicles fitted with a servo.

Regards.