After replacing the master cylinder my brakes don't work
I replaced the master cylinder with a new one thinking the one I bought earlier was defected but I still cannot get the system to bleed. I will get a full pedal for a few seconds and then they go to the floor again. What am I doing wrong?
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Re: After replacing the master cylinder my brakes don't...
Some vehicles require pressure bleeding, for this you need specialised equiptment. some thing you can try is to bleed your master cylinder first. to do this you place the unit in a vise. fill both resevoirs then pump the unit by placeng your fingers over each outlet ani pushing the centre piston with a suitable screw driver.
do this until there is no bubbles left in the system
remove from the vise put into car and bleed normally
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Had the exact same issue with an 04 Cadillac srx. Replaced the booster twice...finally put my old "defective" master cylinder back on and brakes worked fine. Turned out the new master cylinder was bad...even though the symptoms point to the booster. Do check the vacuum hose going to the booster first though. Have seen those go bad and cause weird problems and it's the easiest thing to check first.
Did air get into the master cylinder? If you can, try bleeding the master cylinder. The fact that you aren't getting any fluid out of the line suggests that the master cylinder isn't pumping fluid.
Master cylinders are (usually) on a split circuit system - that is, the front left brake is on the same line as the rear right. The front right brake is on the same circuit as the rear left.
A point to be aware of is when bleeding brakes/doing brake work and if the master cylinder runs dry, is that a seal inside the master cylinder can fail/twist over and not pressurise that particular line, even though it may pressurise the opposite brake line.
I'm in the UK. I have a 1998 Jeep Grand Cherokee 4 litre, which I am slowly rebuilding. Three weeks ago I fitted new front brake rotors, new front callipers and pads. I've been rebuilding cars for over 40 years now and have done countless brake overhauls.
.. and could I get a hard pedal after the job was completed? I must have bled the brakes three times a day for a week and couldn't get a brake pedal. I phoned a full time mechanic I know - he came and tried to bleed the brakes three times and couldn't get a hard pedal.
That's when you think 'The master cylinder was working before I started the job so it must be still working now'.
I fitted a brand new master cylinder and because some parts for older jeeps are hard to come by in the UK, I fitted a second hand proportioning valve. When I bled the brakes again the pedal became rock solid. Fixed.
You're probably fed up with your braking problem now ... and apparently this 'no fluid/brake pedal' is more common than you may think.
Try bleeding the master cylinder if you can. If you suspect that it isn't fully working - try an overhaul kit which is cheaper than a new master cylinder.
As for changing the proportioning valve .. that's your decision. My brakes now work though the truth is I don't actually know which was at fault as I changed both the master cylinder and proportioning valve at the same time. Maybe both were at fault, for whatever reason. I genuinely don't know.
The positive aspect is that you are doing your own work and know what you're doing. You're saving on labour charges.
Un bolt the master cylinder but don;t disconnect the brake lines move to the front unbolt the 4 bolts holding the booster cortrol unit disconnect the main vacuum hose from booster unit take out replace with new one or junk yard unit there a lot cheaper and reinstall in reverse
Your problem look like if there is still some air in the calipper cylinder.
I suggest you loosen the brake hose at the calliper and completely retract the piston.
Then thighen back the hose and perform a normal bleeding procedure of the front brake lines.
Are you losing any Brake Fluid? Is it visible anywhere at the four wheels? Is it visible anywhere else?
The Master Cylinder may be defective. When you bled the Brakes did you maintain proper Brake Fluid Level in the Master Cylinder? If you didn't: and air entered into the Master Cylinder there maybe air within in the system in the Master Cylinder. You can bleed the Master Cylinder by removing the brake lines, adding more fluid, pumping the brakes, bleeding the air out of the system.
Are you losing Brake Fluid and can't find a noticeable leak? Then there is a good posibility that the rear seal in the Master Cylinder is leaking and the Fluid is going into the Brake Booster - which is the large wheel shaped drum located on the fire wall. In this case both the Brake Booster and the Master Cylinder will have to be removed and replaced. The Booster contains seals that Brake Fluid is very caustic to and will ruin these seals- causing future failures.
If no Brake Fluid is lost, no leak found: Check the Brake Booster Vacuum Advance. This should be connected to the outside of the Booster, attached to a hose, with the hose going to the engine. Check to see if this is working properly. Replace if needed.
I had this same problem on my 90 Jetta but it was the master cylinder that was the problem, I first replaced the master cylinder because it was leaking badly. The new master cylinder I put on was a manufactured defect, the internal piston inside the master cylinder was not machined enough (10mills too long) so when you applied the brakes the piston would not move far enough to release the brakes but after 20 mins they would slowly release themselfs just as you said your brakes do. The only thing that can cause this problem (if you have changed everything) is the master cylinder or the brake pedal itself, did you do anything to the brake pedal or near it to change its movemment ? Make sure the brake pedal once depressed returns fully to the float position where it should be. I would try another master cylinder preferrable a used one from another vdub the same as yours, eg year,model and country of origin some parts are different if it was made in Mexico or Germany. Its going to be a pain in the *** to have to replace the master again but I see no alternative. There is one more thing you can check I just thought of....check all your brake lines from the master to the calipers for any signs of dents or sharpe curves if the metal brake line is bent too much it will restrict the flow of the fluid returning to the master cylinder after brake pedal release but not during braking. Good luck
i have ran into this problem , but on earlier gm products---check the local dealership or a well known mechanic shop that has all data and check for bullitains. what i had found out was there was some trucks that gm had installed the wrong power booster . not saying your booster is to blame, but may be a great place to look seeing you replaced abs modules ,master cylinder ect---hope that helped out!!
Did you bench bleed the master cyl. before installing it? then when you installed the new master, did you bleed all 4 points? If yes, then check all for points for leaking caliper/s and leaking wheel cylinder/s. If the van has ABS, check the valve body for a bleeder screw and see if air bleeds out of it. If you've checked everything twice and did everything right then your new master cyl. may be defective.