My dispatch has developed a fault on the brakes. Always had a bit of extra travel on brake pedal but last week pedal went alway to floor. O/s/f & n/s/r brakes reading 80 & 90 on MOT brake tester n/s/f & o/s/r reading 320 locked and 220 locked. I have replaced load sensing valve (new) and master cylinder (2nd hand) I have re bled system and without engine running pedal is hard as soon as I start vehicle pedal goes alway to floor again with very little effort. I have cracked all bleed screws and when I press pedal fluid comes out of every cylinder. I have done MOT test on servo pumped pedal until hard then start vehicle n pedal goes soft.

Garry Wilson, January 2012

Check number one start the engine so that the servo has a full vacumm, turn off the engine and get someone to press the brake pedal slowly down as you listen under the bonnet to see if there is a hissing leak of air from the servo cylinder, If there is check the seal were the brake master cylinder joins the servo cylinder. Also check that the servo pump is pulling a reading of about 640 vacumm using a mitivac and that all the air vacumm pipes run by the servo pump . If the servo is hissing it is time to change the servo cylinder and just to make you day to get it out you normally need to lower the engine.If this is not the fault I take it that you have checked inside the rear drums to make sure that there is no leaking on the brake cylinders. I suspect that the problem will be the master cylinder, yes I know you have changed it but they have a problem that if you push back the caliper pistons faster than super dead slow then the internal seals turn over inside the housing. Get a new one the faults you are discribing is caused by a faulty master cylinder. the reason for the loss of brake pressure on two opposite wheels is down to one of the internal seals failing in the master cylinder.

arizona kid, January 2012