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Old 11-17-2021, 12:37 AM   #1
deadken
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Default Soft brake pedal, only thing left is the ABS module

I had my 1992 944T off the road for a few months while the weather was hot and I was working on other things (the AC isn't working). After at least 6 months, and more like over 9 months, when I went to use the car it had a soft brake pedal. I figured that the master cylinder went bad and ordered a brand new Ate master cylinder (no China junk) and installed it. I bled the master cylinder and found that I still had a soft pedal... Figuring that I somehow got air into the system, I bled the entire system going from R/R to L/F and still had a soft pedal. I figured that it was my helper who was inexperienced in bleeding brakes. I know it is not rocket science, but what else could it have been? So, I used my vacuum pump and then went to gravity bleeding. Over a quart of DOT 4 fluid later and I still had a soft pedal. I went as far as to check the metering / distribution block in case it had a reset able proportioning valve like the old US cars had. It doesn't, and everything looks just fine.

So I ordered a black label Motive power bleeder (machined aluminum cap, not plastic) and plenty more DOT 4 brake fluid. Even though I have NEVER needed to use a power bleeder before, I have heard great things about them. I've gone through over 2 more quarts of brake fluid and I still have a soft pedal!

The only thing that I haven't done is change out the ABS unit. I figure that brake pressure must be leaking into the accumulator even when there isn't a need. I happen to have a 1994 parts car (the same car I am planning on harvesting the R-134 AC system from), so I figured that I'd swap that into the car and see if that corrects my problem.

What I'd like to ask you guys is has anyone else had a problem like this and was the ABS unit to blame?


TLDR: Soft brake pedal after car sat for months and months. Still have a soft brake pedal after replacing the master cylinder and repeatedly bleeding the system. I figure it must be the ABS unit. Has anyone else experienced anything like this?
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Old 11-17-2021, 12:50 PM   #2
cmsuter
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Brake Booster?
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Old 11-17-2021, 12:50 PM   #3
dl242gt
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Before I would convict the abs unit. I'd ask you about the flex brake hoses? Are they original? If they are then you may have issues with the hoses decaying internally. In my own experience I've never been able to get as firm a brake pedal with an abs car as my older 82 non abs car. Still very good brakes but not the firm confidence inspiring pedal my old car has.
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Old 11-17-2021, 01:27 PM   #4
ZVOLV
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Maybe go try to create an ABS event. There isn't a scan tool method on these cars.
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Old 11-18-2021, 01:17 AM   #5
deadken
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Thanks for the replies. You guys make some good points:

1) Brake boosters make the pedal softer, I don't see how this one would start to work better than from the factory.

2) The previous owner replaced both rear hoses with stainless steel braided hoses. The fronts are rubber, and I'm not sure if they are original. Maybe I'll swap them over to stainless steel just to make them match the rear and to rule them out. However, I'd be really surprised if one or both expanded enough to soften the pedal as much as I've got.

3) I was thinking about that. However, I just put on some brand new Michelin all seasons and there isn't any rain or snow around here lately. To make matters worse, I did drive the car into work for a day or three just to see what happened. Unfortunately, someone stopped short in front of me and I almost rear ended them. No ABS event, just not enough pressure to actually stop quickly.

The way I see it is that the accumulator, which is where the brake pressure is bled off into during an ABS event, is allowing pressure to bleed into it during normal applications. I don't see how forcing an ABS event will stop that from happening. But, if I find a sheet of ice in an empty parking lot, you can be sure that I'm going to give it a try!

At this point, I'm off from work next week. If the weather is decent, I'll swap the ABS unit from my 1994 parts car into my 1992 car and see if it makes a difference.

Ohhhh damn... My parts car is a non turbo. I wonder if they have a different brake layout from a turbo car (I can't remember if the ABS swapped sides like the air box and battery did).
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Old 11-18-2021, 01:56 AM   #6
ZVOLV
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I use a large shop air powered Mitivac Fluid Evacuator.

We also use a BG flush machine which pressurizes the reservoir AND vacuums from the bleeders.

I bet if I used the shop equipment mentioned, I would be able to get a firm brake pedal.
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Old 11-18-2021, 05:31 AM   #7
deadken
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After using at least 3 quarts of brake fluid, I think you wouldn't. However, I don't know that until I change over the ABS unit.
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Old 11-19-2021, 04:47 PM   #8
cmsuter
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Brake boosters don't necessarily make the pedal softer. If the diaphragm is torn it can pause the pedal to be very soft. I just ran into that exact issue. Swapped it out and the pedal firmed up.
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