how do the brakes work...?

[peaky83]

Right... have replaced brake lines, calipers, brake hoses, brake fluid and master cylinder...

The brakes still need the pedal pressing quite far before they do anything and even when they do they don't seem that convincing...

I have noticed when i press the pedal I can hear a hissing sound or s shush sound with the engine on or off (I know ite prettty normal with the engine off or at least i think it is...)

So im thinking that maybe my problem is related to the brake pedal somehow? but I don't know how they work... ?   

[peaky83]

?? 

[Horney]

The servo is vacumm assisted. Check there's no leaks. Lso have you bled the system correctly with the bias valve open?

nick

[peaky83]

Is the valve open when the car is low or would it be open when the car is high (As it was originally I mean)

I believe the valve adjusts the bias depending on the weight in the car?

[Horney]

I can never remember which way but the haynes explains which way to push it to open it up for bleeding.

nick

[jezza16v]

Sounds like you've still got some air in the pipes. The brakes work by hydraulics, the brake fluid is essentially incompressible, if you get air in the pipes it just squashes before you get any pressure to the brakes. The detail of how the brakes work is not that complicated, basically as you press the pedal the fluid is pressurised in the master cylinder and that pressure is transferred into the front and rear brake cylinders through the pipes, as its a fluid the pressure must be the same throughout the system, the wheel cylinders are a much larger diameter than the master cylinder so for the same pressure in the system you get, I would guess about 6 times the force on the front pads and say 3 times the force on the rear pads, the drawback is that to get that force you have to move the pedal much more than the brake cylinders move at the discs. The brake cylinders actually move about 0.5mm for discs which equates to about 20mm at the slave, which by the time you allow for about a 3:1 lever ratio at the pedal say, your foot has to move say 60mm plus a necessary bit of slack. Even a tiny amount of air in the pipes can make a huge difference to the pedal travel. The servo helps your foot by using the engine vacuum to add some extra pressure to the pedal. (I've ignored the brake pressure compensator for simplicity)
That any use to you?  

BTW, The car has to be level for the bias valve to be fully open, in other words best to bleed the rears when the car is not jacked up at all.

[peaky83]

Hmmmm thanks for that, very useful. Is it a 2 man job to bleed brakes? I hate doing it, I tried once and it all went very wrong! Anyone live in cheshire?? Fancy helping me?? 

[jezza16v]

If you don't have an special bleed kit then its essential really to have two people, one to press the pedal (usually the wife in my case) and the other to shout instructions and open and close the bleed nipple at the right time and catch the fluid. Remember to keep the reservoir topped up to the minimum mark, if you let it drop too low you'll have to start all over again. Make sure you have plenty of fresh fluid available, don't put back the stuff you bleed out.

[weazgti]

Did you do it wheel furthest away from master cylinder?
Start at the rear furthest away and work round like this

[cняis]

you can do this with the car jacked up, cant remember which way you move the bias valve arm but it needs to be 'open' (i.e as if the suspension was bottomed out) to allow the fluid through properly to the rear brakes. also you can pin the arm into position by lining up 2 holes, 1 on the arm and 1 on the valve iirc.