brake bleeding help

[benlg81]

Hi All

my folks have replaced the rear clipers and rebulit them on their audi cab m reg.

before the rebuild the old calipers piston came out

so new calipers rebuilt all put back together and on the car they have bled the brakes 4 times now and have all clean fluid coming out.

get to the point where the pedel is nice and hard but when you start the car the pedel pressue drops and the pedel goes to the floor

any ideas i know it will be a stuipid easy answer but we are stuck

Regards

BEn

[vag80]

the brake master cylinder may have gone,some time when you bleed the brakes if you have drained all the fluid the seal in the master cyilneder fail

[benlg81]

cheers mate

any way to test this?

do you need the ignition on or off to bleed these brakes?

[vag80]

if the master cylinder has a bleed nipple start by bleeding the brakes there first ,there is no real way of testing them that i know (but i may be wrong on that) i don't matter if the ignition is on or off,
how did they bleed the brakes?

[Rickman]

Abs = ignition on...
Press and hold the pedal and the abs pump will do all the work, undo a nipple and the fluid will flow fast and free.

Start with the furthest brake from the master cylinder and work your towards.

If you start the car with your foot on the pedal, then it will hit the floor..

Check the pipe from the master job to the inlet, make sure its fitted correctly and not leaking

[benlg81]

right then it is the master cylinder

anyone got one spare they want to sell?

[benlg81]

right

new master cylinder all fitted and bled and still no pressure in the brakes you have to pump to get the pressure back

any ideas what else it could be?

[vag80]

you my have a problem with the servo (booster) that the master cylinder bolts to

[benlg81]

anyway to test this out?

[urS6]

Originally Posted by benlg81

... the pedel is nice and hard but when you start the car the pedel pressue drops and the pedel goes to the floor

You expect the pedal to sink a bit when the engine is started as you get a bit of assist (either vacuum assist or hydraulic assist, depending on model).

If the brake pedal is fine when the engine is off, doesn't need pumping to build pressure, doesn't sink slowly to the floor, etc., and only starts misbehaving when the engine has started, then it must be either the master cylinder (seals gone), or the servo assist (Haynes manual should suggest a way of testing).

Are you sure it's completely bled? Did you use a pressure bleeder (or a vacuum bleeder)? If you don't plug the pipe properly when you disconnect something like a rear calliper, you can get air bubbles way inside the pipes. Normal bleeding might not shift the bubble, and the fluid will run clear when it comes out of the bleed nipple. Extended pressure bleeding (could be talking about running a litre of fluid through) should do the trick.

Paul