Brake hissing booster problem

[ruswin]

Hey all... new to this forum, a Supercoupe convert from the SCCOA. Just picked up a sweet 1994 Markviii with 56,000 miles on it. It has a problem with the brakes that I think is the brake booster but I want to be sure before I tear into it. I have looked through A LOT of threads here but did not find the exact description and remedy of the problem.
When you press on the brake pedal, the pedal moves and I can hear a hissing sound inside the cabin, the car BARELY stops. The car also starts to stutter a bit and run rough when the brakes are pressed. I checked the check valve on the front of the brake booster and it is fine. So I assume I am looking at a bad booster. Did I miss a thread on the procedure for replacing the brake booster? Looks pretty easy but I am sure there are shotcuts. If I am only replacing the booster, will I have to get into the hydraulics at all or worry about bleeding the brakes?
The other question I have is about the ABS light. Would it be ON if the booster is no good? The ABS light is also on and I am not sure if there is another gremlin in there. Any help is appreciated.

 

[whiplash15]

I think you hit the nail on the head. Ruptured daiphram in the booster.

 

[Aquabrit]

Could be a bad booster. I thought i had a bad booster last year and replaced it - turned out that the rubber seal between the booster and the master cylinder had disintegrated, so you might want to check that first.

 

[ruswin]

Ok, I did a little more checking. When I loosened the 2 screws that hold the master cylinder to the booster, I could hear air escaping from it. That is after sitting idle for 24 hours. If the rubber seal between the master and booster was the problem, I assume I would not have heard the air escaping and that it is indeed the booster diaphram that is no good. The fact that pressing on the pedal causing the engine to idle lower RPMs and rough seems to mimmick what happens if the manifold vacuum line is disconnected from the booster.

So what do you all think... bad booster for sure or what?

 

[Aquabrit]

Looks that way. Sounds like the booster is maintaining vacuum, just not working. When i was testing mine, if i pulled the check valve out or disconnected the master cylinder there was nothing - no hiss etc. So yeah, sounds like your booster's diaphragm is shot.

 

[ruswin]

Thanks blue on blue. Yeah sounds like the booster... let the fun begin. Am I going to have to do the extended brake bleeding after replacing the booster? I noticed that the hydraulic connections to the master cylinder need to be disconnected in order to move it forward and out of the way. Never really messed with an ABS braking system with an extended bleeding procedure that sounds complicated.

 

[Aquabrit]

If you are careful, you can pull the master cylinder away from the booster enough without disconnecting the lines.

 

[ruswin]

So I bought a used brake booster and it is missing the sensor and sensor port that is on my original booster. My question is 2 fold.

1) What is the sensor that extends from the right side of the booster? Is it necessary?

2) Is the booster on my 94 Mark the same as the booster on a Thunderbird and / or Cougar from the same year? I see that some boosters from that year do not have the ABS option? I thought all MN12 bodied cars from that era had ABS?

FYI... I have already removed my booster from the car. It was not too bad of a job. DEFINATELY remove the wipers, cowl cover, wiper motor and then it is easy to get to. I did not even disconnect the 2 steel brake lines from the master cylinder, just carefully moved them as I disconnected the master from the booster.

What looks like it will be a pain in the A#@ is putting the brake pedal connection back together and installing the 3 nuts that hold the booster to the fire wall. I am tall and have long arms and big hands which makes this job pretty hard to do. Any suggestions to how to make it easier to reinstall? Once I am done, I plan on posting a step by step procedure to replace it.

Thanks

 

[turborich]

Mine is showing the same exact symptoms! I figured it was the booster as well, just wanted to do a search first & see what all was involved. Pretty much what I figured, remove the cowl/wiper unit. Sounds like fun!