by "Flynorcal: pilot, offshore sailor, car racer and panty thief" (flynorcal)
Published 06/22/2017 at 18:08
by "Flynorcal: pilot, offshore sailor, car racer and panty thief" (flynorcal)
Published 06/22/2017 at 18:08
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I’ve noticed something odd, and have two guesses but want feedback from those who might be better informed.
1992 Corvette loses brake pedal height while on the freeway. I don’t touch the brakes for long periods of time and the pedal is low when I finally get around to using it. One pump and it’s back in action. Stays firm, can’t push it to the floor, no noises or any other symptoms. Brake fluid level is correct.
My guesses are that the master cylinder is going bad, or I have a vacuum assist of some kind that might be failing? Or the vacuum system has a leak and loses suction somehow?
I don’t know how the vacuum assist thing works or even if I have one. I just feel like it’s more likely the problem as the master cylinder malfunctions I’ve experienced have all let the pedal drop to the floor or feel really sponge like.
Any guesses?
Only happens while freeway driving. Never at a stop light or during city driving.
"PS9" (PS9)
06/22/2017 at 18:18, STARS: 2
Well too bad, because you’re gonna get bad information from people who are poorly informed.
I suggest replacing the whole braking system twice and filling the lines with whipped cream and crazy glue each time.
"I have another burner, try to guess it!" (ihaveanotherburner)
06/22/2017 at 18:24, STARS: 1
I think your on the right track w/ it being the MC, if it was the booster the pedal would get harder.
I’d undo the MC from the booster (don’t need to undo the brake lines) and check for fluid on the back of the MC.
"I have another burner, try to guess it!" (ihaveanotherburner)
06/22/2017 at 18:25, STARS: 1
My guess for why it only does it on the hwy is bc in town your expelling the small amounts of air before they’re big enogh to notice.
"CaptDale - is secretly British" (captdale)
06/22/2017 at 18:32, STARS: 2
Assuming it is an LT1 (LT5 is different) you have a single diaphragm booster which uses vacuum. This is early ABS too and that is always hard to diagnose, but it does sound more brake master than anything else. I would start there. Maybe a rebuild kit if a new one is cost prohibitive.
"crowmolly" (crowmolly)
06/22/2017 at 18:47, STARS: 1
Yeah, sounds like it is a failing master.
"Flynorcal: pilot, offshore sailor, car racer and panty thief" (flynorcal)
06/22/2017 at 18:52, STARS: 2
Clearly you’re one of the previous owners based on some of the other work I’ve found on this car
"ITA97, now with more Jag @ opposite-lock.com" (ita97)
06/22/2017 at 19:53, STARS: 0
While you’re replacing the master cylinder, it will give you the perfect chance to pop in a new brake bias spring and bring the rear brakes to life. If you want one, I never did get around to putting the DRM spring in my own C4 before I sold it. I think it is still sitting on my workbench. I’d be happy to mail it off to an oppo.
"MM54" (mm54mk2)
06/22/2017 at 20:23, STARS: 1
I suspect pad knockback. Make sure your wheel bearings are good and caliper slides are well-lubed and free.
"Flynorcal: pilot, offshore sailor, car racer and panty thief" (flynorcal)
06/22/2017 at 20:28, STARS: 0
Oh by all means! Yes!
My email address is oppo at noops dot org.
"Flynorcal: pilot, offshore sailor, car racer and panty thief" (flynorcal)
06/22/2017 at 20:32, STARS: 0
That’s a super solid suggestion. The wheel bearings seem fine and the front calipers look pretty good but I’m gonna pull the wheels and get a better look. I’d never heard of such a thing but it makes sense and fits the symptoms perfectly.
"Flynorcal: pilot, offshore sailor, car racer and panty thief" (flynorcal)
06/22/2017 at 20:50, STARS: 1
Having given this a bit more thought there was one other thing I noticed which supports this idea. The pedal is gets low but works exactly the same except it starts lower (and heel toe is out of the question) but grabs and works. It just feels weird that the pedal height isn’t as tall as expected.
If this is the case I’m pretty sure when I’m on the track I’ll get it after a few turns with light or no braking, scaring the life out of me when the pedal isn’t where I left it. Giving the brake a tap left my left foot before the braking zone should also make it go away.
Easy to confirm the problem if not solve it.
"ITA97, now with more Jag @ opposite-lock.com" (ita97)
06/22/2017 at 21:29, STARS: 0
I may have spoken too soon. Looking on my work bench, I can’t find the envelope I put it in, but here is where I got it:. http://dougrippie.com/products/drm-brake-bias-spring-3/
However, I did find that I still have one new and one used but still probably serviceable thrust washer for the rear wheel bearings of a C4. They’re the Teflon coated thrust washer that sits between the rear hub and the axle stub, and they are damn near impossible to find (and if you’re tracking your C4, you’ll get to replace rear hubs on a regular basis). As a consolation, I’d be happy to stick those in the mail to you.
"MM54" (mm54mk2)
06/22/2017 at 22:27, STARS: 1
My crown vic does the exact same thing, after spinning the wheels or making very hard turns, the brake pedal is very near the floor the first pump or two. I spent about a year chasing it around, all I can figure now is that the excessive endplay (axial) in the axle shafts from the super-worn clutch packs in the diff are pushing the rears more than the slides can cope with.
"Flynorcal: pilot, offshore sailor, car racer and panty thief" (flynorcal)
06/23/2017 at 02:12, STARS: 0
I didn’t know it was that bad -- I heard the wheel bearings were a source of pain but most of the common complaints about the C4 seem to be a little exaggerated. Unlike Porsche. Yeah if you have parts you aren’t going to use and think I might, I’ll do my best to put them to use. Shoot me an email!
"ITA97, now with more Jag @ opposite-lock.com" (ita97)
06/23/2017 at 07:34, STARS: 1
On street cars, the rear bearings last about 50-60k. On mine, one side was completely shot when I bought the car with 55k on it, and the other side started to develop a bit little bit of slop a few thousand miles later.
There used to be a guy in the region who raced a 93 40th anniversary package car in ITE. Running sticky rubber, I think he used to change rear wheel bearings every other race weekend. The good news is that they are pretty quick and easy to do.