Sabre Posted August 19, 2018 Share Posted August 19, 2018 We recently replaced the brake fluid in my son's 300 TDi Discovery. He drove the car for a week after that, and the brakes were good. The car was then parked for a week, and when he drove it again braking action was very bad. You really have to step on the pedal to slow down the car. It feels as if there is no power assistance We bled the system, and a lot of air came from both rear wheels. But, no change to the brakes. We bled the system again, and a few bubbles came from the left rear brake. All other wheels bled clear brake fluid. But there is still no change Is there a possibility that the brake booster could have failed while the car was stationary ? Is there a way of testing the booster ? Any tips on bleeding the system ? I have bled many brake systems on many vehicles in my life, including my own 300 TDi Discovery, but have never come across this issue before. Quote Link to comment Share on other sites More sharing options...
ThreeSheds Posted August 19, 2018 Share Posted August 19, 2018 This has me stumped... How are you bleeding the brakes? Using a vacuum pump by any chance? Quote Link to comment Share on other sites More sharing options...
Ex Member Posted August 20, 2018 Share Posted August 20, 2018 Check that you are getting vacuum from the pump and check that the booster is holding vacuum. Both fail commonly. 1 Quote Link to comment Share on other sites More sharing options...
HoSS Posted August 20, 2018 Share Posted August 20, 2018 If you have not bled correctly or a leak the brakes will be spongy. If you have lost vac assist they will be firm but need a lot of foot force. Different symptoms. 1 Quote Link to comment Share on other sites More sharing options...
Eightpot Posted August 20, 2018 Share Posted August 20, 2018 The brake boosters can get tiny stress cracks around the master cylinder mounting studs - it might be that all the pedal pumping to bleed the brakes has opened one up. I've had success using good rtv sealer around the studs. 1 Quote Link to comment Share on other sites More sharing options...
mmgemini Posted August 20, 2018 Share Posted August 20, 2018 As Eightpot says. Check Quote Link to comment Share on other sites More sharing options...
markyboy Posted August 21, 2018 Share Posted August 21, 2018 Those symptoms sound like a faulty servo or vacuum pump. To test the brake servo, pump the pedal a few times with the engine off to get rid of any residual vacuum. Apply the brakes with the brake pedal then start the engine. If the servo is working, the pedal should sink further down toward the floor when the engine starts. If the pedal does not sink any further, either the servo is faulty or the vacuum pump is kaput. The vacuum pump could be tested by removing the hose from the servo and placing a thumb/finger over the end of the hose to check for vacuum. 1 Quote Link to comment Share on other sites More sharing options...
Sabre Posted August 21, 2018 Author Share Posted August 21, 2018 Thanks for all the replies. After reviewing all he replies, and the fact that the vacuum pump leaks like a sieve, my son decided to replace the vacuum pump. We will order a new pump tomorrow Quote Link to comment Share on other sites More sharing options...
Sabre Posted August 21, 2018 Author Share Posted August 21, 2018 On 8/19/2018 at 9:53 PM, ThreeSheds said: This has me stumped... How are you bleeding the brakes? Using a vacuum pump by any chance? We used the old method, pump brake pedal, hold, open bleed nipple to allow air/brake fluid to escape, close bleed nipple Quote Link to comment Share on other sites More sharing options...
Sabre Posted August 28, 2018 Author Share Posted August 28, 2018 Some (egg-on-face) feed back We wanted to replace the vacuum pump on the system on Saturday. Whilst looking around the engine bay, my son asked : "What does this pipe do ?" It was the vacuum pipe from the vacuum pump going to the booster. The pipe was pulled off, and sat right in front of the booster inlet, about 5mm from it Connected the pipe, and the brakes were 100% 😎 On 8/19/2018 at 9:53 PM, ThreeSheds said: This has me stumped... How are you bleeding the brakes? Using a vacuum pump by any chance? We used the old method, pump brake pedal, hold, open bleed nipple to allow air/brake fluid to escape, close bleed nipple 2 Quote Link to comment Share on other sites More sharing options...
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