Ok, Double check your bleeders on the brakes and make sure they are tight and not letting air into the system. Make sure the fluid level is full in the reservoir and covering the master cylinder pistons. Did you get steady streams of fluid when you bled the brakes or were there bubbles in the fluid there as well? It takes several tries to get all the air out of the lines when doing this so be patient and bleed them again if needed. Nothing electrical is involved in this so don't worry about fuses and such. If after bleeding again and all air is out of the system and the problem still is there then the master cylinder will be suspect and it must be leaking by and will need replaced. I assume you have looked all the brake lines over and saw no leaks in the lines as well, and that the wheel cylinders and calipers were inspected for leaks and bad seals. And after installing new brakes they were adjusted correctly so there is not too much travel on the wheel cylinders.
How are your brakes set up, disc brakes in front and shoes in the rear or 4 wheel disc or all shoes. Let me know what you find. Thanks
I just now can see the comments posted earlier. If the brake booster is leaking anything into the system it is getting in past the push rod on the master cylider and body seal which will reguire a new or rebuilt master cylinder. It does not actually add vacuum to the master cylibder it assist the pedal movement before it goes into the master cylinder so the leaks that I need investigated still need clarification. I suspect rear wheel cylinders may need the brakes adjusted. and they could be allowing too much travel. Many times people think that automatic adjusters will adjust the rear brakes. Unfortunatly they need to be manually adjusted first. You need to tighten them up until the wheel can not be turned. this insures that all the components are set correctly, then you back them off until you feel very little or no drag.
If the brakes are not properly adjusted, the wheel cylinder cups will travel beyond their normal range - into the rusty area. You may get away with resetting the normal range, however I would suggest changing them.
As usual, lube all the moving parts with never sieze or suitable lubricant, backing plate pads, adjuster asm, emergiency cable etc.
Sorry for all the comments but its been a long night and I forget things till its too late to add them. But even if the booster was overcompensating the assist you would still have a hard pedal if there were no leaks and the brakes were adjusted correctly. Once they lines build up pressure, it will stop the pedal or blow out a line if the pressure is too great and the pedal travels too far.
Its pssible you have something in the booster that is magnifiying the issue and if you have brakes without the booster but pedal goes to the floor with it, there still is an issue with overtravel that has to be explained. Your absolutely sure there is no leaks correct? Have you actuall tested the rear brakes, or did you just adjust them? Can or do you feel safe driving it a very short distance to see if the rear brakes are working, Both with the booster and without? Almost always when an issue like this is occuring it involves the rear brakes and cylinders along with the adjustment. I hate for you to get a new booster till we resolve everything else and if we can't find an obvious cause then it has to be the booster, that is a very uncommon issue but it does happen. How did you bleed the lines? With a vaccum bleeder or manually? I recommend using a vaccum pump.
If you did, and all the air is out of the system, then possibly you have a siezed up caliper slide. Do all the calipers slide freely? Make sure all the sliders including the rear ones move freely. That will cause a soft pedal.
Also did you mess with the pushrod adjustment when you removed the mastercylinder. (out by the fire wall under the hood?) You should not have. One last thing to check is the brake pedal bracket. They sometimes crack and cause a soft pedal.
Oh yea and check the master cylinder push rod adjustment under the steering colmun. Is is adjusted correctly.
In general a bad booster will cause a hard to push brake pedal that does not go to the floor. A bad master cylinder or leak in the brake system will cause a soft pedal that goes to the floor.
A way to test if the booster is holding vacuum is with the engine off you can:
1) Press the brake pedal a couple times, it should become progressively harder to push, once the car is started back up with in about 30 seconds of a smooth idle the pedal should have a normal feel to it.
2) Wiggle the plastic fitting that connects the vacuum line into the booster. Be gentle so you don't break it but pushing it to the side a bit breaks the seal and you should hear a definate hiss. This hiss tells you it is holding vacuum and is good.
3) This one is kind of redundant but I had to do this to a friend to prove my point once. You can disconnect the vacuum line to the booster (and plug it off) start the engine and after the first one or two pushes of the pedal it will be hard to push. This simulates what a bad booster feels like, plugging the vacuum line back in it will return to normal pedal effort if the booster is good.
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Raise the vehicle and check each wheel to see if the brakes get tight. If one is not check the rubber brake hose for it maybe collasped causing this problem
Also to remove all air from the brake lines you start at the furthest wheel from the master cylinder and work your way up to the closest wheel last. If you did not do it in this proceedure air will still be in the lines
Fig. 1: Loosen the front brake line in order to bleed the master cylinder
Fig. 2: Connect a bleed hose from the bleed valve on the front caliper to a jar of brake fluid
Fig. 3: Always follow the lettered sequence when bleeding the hydraulic brake system
Hope this helps to solve it; remember to rate this answer.
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ftw1952,
This was done before, but did it again. Get some braking with engine off, but straight to the floor when its running.
John
I had already tried those things, but did them again. The results were the same - some brakes when motor not running and to the floorboards when it was. Think it must have something to do with the assist, but don't know what.
John
Have done all you suggested including a new master cylinder and have bled to the tune of 3 litres brake fluid. Took thr hose off the vacuum assist, holding finger over the hose and the brakes remain;
removed the 1 way valve and found it not working; replaced that, started the engine applied the brakes and to the floor the pedal went; stop the engine and the brakes return. About ready to drive it off a cliff...
John
Thanks for your latest.Will give it all a check in the AM.
John
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