Brake Booster
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Elder User
Joined: Oct 2003
Posts: 986
Likes: 0
From: Gulf Shores, AL
Brake Booster
How much vacume should I have ging to the booster? My brakes are soft. I can push the pedal to the floor wihout the fronts even thinking about locking up. I have 35s A/Ts on it but I think they ought to work better than they do.
Lead Driver
Joined: Nov 2002
Posts: 9,557
Likes: 44
From: Concord, NC
Sounds like you have an hydraulic problem with the brakes, not the booster. If the booster failed, pedal effort would be much higher for the same braking force. From your description, braking force, not pedal effort, is the issue.
To answer your question, though, the booster should get full engine vacuum, which should be in the neighborhood of 20" at closed throttle.
To answer your question, though, the booster should get full engine vacuum, which should be in the neighborhood of 20" at closed throttle.
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Elder User
Joined: Oct 2003
Posts: 986
Likes: 0
From: Gulf Shores, AL
No not that I notice because I can push it to th floor with resistance, but it is very spongy and I have bleed and bleed the brakes and no air has come out. I do not loose fluid, and I have installed a new master cylinder. I can push the pedal to the floor and the best analogy I can think of is is feels like a have a large trailer pushing me. It does not want to stop. The booster has about 300K miles on it also. I am at a loss. I realize that with 35s I will loose some breaking but this is an abnormal amount. The only thing left that I can think of is the booster. Any other ideas?
Post Fiend
Joined: Jan 2005
Posts: 8,402
Likes: 10
It is not the booster. Sounds like you have air in the system. When you fitted the new master cylinder did you bench bleed it? When you bled the brakes did you bleed the RABS valve unit after the rears and before the fronts?
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Elder User
Joined: Oct 2003
Posts: 986
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From: Gulf Shores, AL
I bench bled the cylinder but not the RABS I was under the assumption it would pass the air. Maybe I need to remove the master cylinder and rebleed it as well as the RABS. I already ordered a new booster as you can hear a small hiss from time to time at a red light. As I said it has 300K plus on it. I will give all that a try. I figured the master cylinder would pass any missed air as well as the RABS unit. I never had a problem with the RABS before.
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Postmaster
Joined: Oct 2008
Posts: 3,026
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From: Renton, WA
Master cyliders do not "pass air" very well, bench bleeding is VARY important. With these Ford masters because you can't really get a line back into the reservior I go so far as to bench bleed it with the body of the master submerged in brake fluid.
If I had learned one thing from this site it is that the RABS is notorious for bleeding problems.
If I had learned one thing from this site it is that the RABS is notorious for bleeding problems.
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Post Fiend
Joined: Mar 2002
Posts: 19,402
Likes: 38
From: Bonita Springs FL
It's definitely not the booster. A bad booster can never cause a soft pedal.
Der Postmeister
Joined: Sep 2006
Posts: 6,859
Likes: 560
From: Long Beach, Calimexiforia
It's been a long time since I've had brake issues but isn't their a proportioning valve hanging off the master that properly proportions front to rear. If so, what happens if the lines got reversed. Any validity to that notion or did I just have another senior moment they keep telling me to get used to?