1998 Trans Am Braking Issue
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From: Toronto
1998 Trans Am Braking Issue Recently I had my car in the shop to get the rear brake lines replaced. They had never been done and I was out driving and basically lost brake pressure because they had started leaking. When I got the car back today I took it for a test drive with the guys from the shop. I was pressing the brake pedal a lot, just to test everything out and make sure it was working right. Pressure and stopping power seemed good but after 10 or so quick presses in a row, it would make this sound I would describe as like a "farting" sound from the pedal. It never did that before but I don't think i ever really tried to test out the braking like that.
The guys at the shop checked everything over and said they didn't see any issues or leaks, and they told me they bled the system after they replaced everything. They think it's just making that sound when I step on the pedal quick like that because the brake booster is depleting after that many presses.
What do you guys think? I'm attaching a video of the sound. I press a bunch of times and it's normal, then a bunch more and you can hear it.
The guys at the shop checked everything over and said they didn't see any issues or leaks, and they told me they bled the system after they replaced everything. They think it's just making that sound when I step on the pedal quick like that because the brake booster is depleting after that many presses.
What do you guys think? I'm attaching a video of the sound. I press a bunch of times and it's normal, then a bunch more and you can hear it.
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From: Schiller Park, ILL Member: #317
My guess is that the brake booster is slowly starting to die (the diaphragm inside the metal shell is failing).
I had a similar sound on my '74 Cutlass last year. I had also already been through the process of doing other brake system repairs (needed a master, prop valve, etc.), but even with all the other repairs that sound continued from time to time, and sometimes I'd feel a shudder in the pedal when I would hear the sound. Braking performance was still adequate for the application (small discs + 4,000lb car, so you can't expect much), but even with all the other new parts it still made that sound. I ignored it for a while, and after about a year that diaphragm finally let go and I had a major vacuum leak (the outer shell isn't sealed, so when the diaphragm fails there will be a big leak).
In my case, I had a rather rare booster (small diameter double-diaphragm unit, usually issued to big block cars of that era), and nobody had a core for one of these so I had to have my original rebuilt by a local shop. But I'm sure there are plenty of booster cores available for a '98 Trans Am. After I had the booster rebuilt and I installed it, that was the end of any noises - and overall stopping power was noticeably better (I hadn't realized just how much braking assist was being left on the table once the booster had begun to deteriorate...I guess it was a slow process).
All of the pumping involved in bleeding the system for the other repairs, and then the hard braking you did as a test has probably just accelerated the deterioration of a booster diaphragm that was already marginal.
I had a similar sound on my '74 Cutlass last year. I had also already been through the process of doing other brake system repairs (needed a master, prop valve, etc.), but even with all the other repairs that sound continued from time to time, and sometimes I'd feel a shudder in the pedal when I would hear the sound. Braking performance was still adequate for the application (small discs + 4,000lb car, so you can't expect much), but even with all the other new parts it still made that sound. I ignored it for a while, and after about a year that diaphragm finally let go and I had a major vacuum leak (the outer shell isn't sealed, so when the diaphragm fails there will be a big leak).
In my case, I had a rather rare booster (small diameter double-diaphragm unit, usually issued to big block cars of that era), and nobody had a core for one of these so I had to have my original rebuilt by a local shop. But I'm sure there are plenty of booster cores available for a '98 Trans Am. After I had the booster rebuilt and I installed it, that was the end of any noises - and overall stopping power was noticeably better (I hadn't realized just how much braking assist was being left on the table once the booster had begun to deteriorate...I guess it was a slow process).
All of the pumping involved in bleeding the system for the other repairs, and then the hard braking you did as a test has probably just accelerated the deterioration of a booster diaphragm that was already marginal.
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From: Toronto
I took it for another drive today. I can reproduce it almost every time if I hit the gas and then right away switch over to the brake. Is the diaphragm something you'd replace by itself or should I just replace the whole booster? If I'm doing this, should I replace the master cylinder at the same time? They're both original, never had anything replaced on the braking system ever other than the rear lines this time.
Last edited by silver98ws6; Sep 26, 2024 at 08:41 PM.
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From: Toronto
A couple more tests I did, if I pump the brakes while the car is off, it gets harder, so that's normal, and then when I start it I can feel the boost kick in, also normal. I also tried all the stuff I was doing while rolling with the car stopped and it doesn't happen, so it only happens while the vehicle is rolling.
Last edited by silver98ws6; Sep 26, 2024 at 05:13 PM.
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From: Schiller Park, ILL Member: #317
You don't have to replace the master at the same time unless it's having issues of it's own (sometimes a leaking master will contaminate the booster, so obviously in that case you'd want a new master too). Just be sure not to loose the pushrod when you pull the two apart.
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Thanks for all your help so far guys, really appreciate it. I got one other question, you mentioned a big leak when your booster failed. Was it a vacuum leak or brake fluid? I'm trying to understand what might happen if I keep going until failure. Would I need a tow or would I still be able to limp it to a shop?
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From: Schiller Park, ILL Member: #317
Thanks for all your help so far guys, really appreciate it. I got one other question, you mentioned a big leak when your booster failed. Was it a vacuum leak or brake fluid? I'm trying to understand what might happen if I keep going until failure. Would I need a tow or would I still be able to limp it to a shop?
When the diaphragm totally fails, you'll loose power assist but the brakes will still function (like an old manual brake setup), so it'll take a lot more pedal effort to stop. The vacuum leak will mess with idle quality but the IAC and STFT adjustment should be able to compensate enough to keep the engine running. On my carb'ed car, obviously with no IAC or PCM control/O2 feedback on fuel trims, the idle was very poor and it was a bit hard to keep running (but it did still run). In my case, as luck would have it, the moment of total failure actually happened once I had pulled into my driveway so it wasn't so bad.
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