Welcome to the Corvette Forums at the Corvette Action Center!

Bad Shock

Tom Bryant

Well-known member
Nov 9, 2000
Edgerton, Ohio, United States
1959 black 270hp (9/2/69) 1981 Beige L81(10/20/80)
Went to Cars and Coffee a couple weeks ago and haven't really looked at the '81 close since then. I wiped it down before the show and everything was clean. Yesterday I noticed a hazy oil film on the left rear wheel. The caliper looks dry so I stuck my phone under the front of the wheel and shot a picture. It appears that one of the original shocks has blown out. Now I have to decide what shocks to put on it. Any suggestions on shocks that you have been happy with? I'd be looking for something that looks stock with original or better performance.

I need to replace the outer spring bolt cushions too so I might as well do it all at once.


p.s. It was real hard to roll the car over on it's right side to take this picture.


  • 1981 Corvette Shock.JPG
    1981 Corvette Shock.JPG
    1.3 MB · Views: 187
  • Thread starter
  • Moderator
  • #2
It looks like the current AC Delco replacement for this car is the 5304 Professional Premium Gas charged shock. It's $23.79 at Rock Auto. I suspect it's the same shock that CC sells for $149.95 in a set of four with hardware. That's not a bad deal but I'm only looking to replace the rears at this time and this car has no rust underneath so I really don't need new hardware either. Since it's my wife's car I'm not looking to upgrade to a high performance shock that would give a firmer ride.

The car is 36 years old. At 25 bucks a corner from Rockauto.com, why not replace them all?
  • Thread starter
  • Moderator
  • #4
I agree. I was just out looking at the fronts and they won't be hard to take off. The upper stud and nut looks like it was just put on yesterday so It should come off easy. I doubt that the originals were gas charged. It would be better to have the same thing all the way around.

I agree. I was just out looking at the fronts and they won't be hard to take off. The upper stud and nut looks like it was just put on yesterday so It should come off easy. I doubt that the originals were gas charged. It would be better to have the same thing all the way around.


I concur. Seems like a cheap fix, although I have no stones to throw. I replaced my back shocks, but not the front. The previous owner put a fiberglass mono spring on the back, and it bounced like a trampoline before shock replacement. I used Monroe, but they're only 3 years old, so I don't know how good they are yet. If I could do it over I'd go with the OEM replacements.
I replaced my fronts with AC Delco a few years ago.
rides nice :thumb
  • Thread starter
  • Moderator
  • #7
I was just reading the ad for reproduction shocks from Corvette USA in the NCRS Driveline.They say that the original shocks only had 10% dampening on compression and had 90% dampening on rebound. That might explain why there is such a sharp shock when you hit a pot hole. Also new shocks have a nitrogen gas chamber that takes up the space not occupied with oil to prevent aeration and cavitation that renders your shock nearly null and void over a washboard surface. They also claim that you should use shocks with the original specs because that's what GM designed the suspension to work with. I don't know why GM used such a weak compression dampening. I would think that about 30/70 would be better to take up some of that initial shock when you hit a bump. I also believe that GM would have used gas pressurized shocks had they been available back then.

You have to call for price on the repop shocks so you know they aren't cheap. I think I'll go with the new AC Delcos and take advantage of modern technology.

I wouldn't put much faith in a report saying that the car had 10/90 shocks on it. Even Driveline can be incorrect.

Think about that 10% valving on compression would have the car bottoming all the time, especially in the rear which lacks travel. Plus a car with what amounts to no compression valving would handle terribly.
  • Thread starter
  • Moderator
  • #9
It does seem ridiculous to use a shock like that but that is what they said in their advertisement. I don't know where they came up with those specs. I'd like to see a Delco speck sheet on the original shocks.

Going back to the old 90/10 front drag shocks we used to run back in the '60s, they had practically no dampening on acceleration and the front of the car would fly up in the air. Then the 90% would hold it there transferring the cars weight to the rear for traction. Given that, you would expect a 10/90 to compress fully under impact and then the 90% would hold it compressed only allowing a very slow rebound. That is not the case when I bounce the car. It drops then rebounds and stops.

  • Thread starter
  • Moderator
  • #10

When it rains it pours. I got the new shocks and they went on easily without any drama. I decided to also replace the rear spring cushions since they were pretty flattened out. That was easy too. I kept looking at the left rear caliper to try to determine if the dampness I was seeing was the shock oil; it wasn't. I pulled the pads out and the inside one was wet. The right side also had one seal on the inside that was showing a little dampness. The fronts were dry. I replaced them a few years ago.

So I ordered new pads and rear calipers. I ordered them from a vendor that usually has my orders to me in one or two days since they are close. Got the pads but the calipers were backordered. No problem. The estimated shipping date was still 2 weeks before the last show of the season I was going to attend. Then the date was pushed back. Then it was pushed back again. They finally arrived the day of the show so I put them on later and finished up everything else except for bleeding.

I was planning on having the wife pump the pedal but I saw a couple other things on Youtube that looked interesting that you could do alone. At this point I had time to experiment so why not see if this stuff really works? I went to loosen the right rear bleeders and holy crap they were overtightened. I have never before bought a new or rebuilt caliper or wheel cylinder and experienced any bleeders that tight. I got out a 6 point 5/16 socket and 1/4 drive breaker bar and got them loose. I wasn't sure they were going to come looses or twist off. They looked ok inside after I got them loose so I put some thick grease on the threads so air couldn't be drawn in during my next experiment.

First method was gravity bleed. It worked on YouTube on a '69. He opened the bleeders and it ran out in a stream. Not on my car. I had them open all at a time or one at a time with the mc cap off. It made no difference. After hours all I got was a drip now and then. It didn't work on my '69 C10 either when I did the disc brake conversion and the calipers are nearly right below the mc.

Next I tried the jar method where you use a length of clear tubing that fits the bleeder tightly and the other end is submerged in brake fluid in a jar. That way you can open the bleeder and slowly pump the pedal and air cannot be drawn back in. Looks like a good one person way to do it. You loop the tubing higher than the bleeder so you can see the bubbles in the trapped fluid. This appeared to really work. I got brake fluid down through the caliper and no bubbles in the tubing after all the air was out. Now the funny part. I could watch the tubing for 30 seconds or so and air would start to come out of the outer bleeder. Every time I thought I had it, all I had to do was wait long enough and here come the air bubbles. I give up . I didn't even try the left caliper.

So I go in the house and recruit the wife to pump the pedal. The right caliper bled quickly on both sides. Then I go to the left rear caliper. If I thought that the bleeders were tight on the right caliper, I hadn't seen nothing yet. I was really sweating getting these loose without twisting them off. Finally they came loose and the caliper bled quickly once it filled with fluid. I hit them both a second time and called it good. It had great pedal and I thought I was going for a pad bedding ride. I cleaned everything up and went to put the rubber caps on the bleeders again. Right inner looked like fluid come out when I put the cap on. I wiped it off and put the outer cap on. Fluid come out around the bleeder so I took it off and cleaned it again. Put it back on and more fluid. I pushed on the cap and I could pump brake fluid out around the bleeder. I watched it and then noticed the fluid was coming out of a crack in the casting along the bleeder bore. Bubba the rebuilder had cracked the casting by overtightening the bleeders both inner and outer. I went around to the left side and found that the outer bleeder bore was cracked there too.

I'll withhold names of the vendor and brand of the caliper until after I talk to them in the morning. More to come.

All your problems stem from rolling the vehicle on its side while your working on it............:)
That just sucks Tom. Hopefully they will provide you with working replacements.
It never rains but it pours. :ugh

  • Thread starter
  • Moderator
  • #14
The nice lady at Summit took good care of me and I hope I'll have replacements on the way shortly. They will be coming directly from AC Delco and I hope they don't get back ordered like the last time.

  • Thread starter
  • Moderator
  • #15

Just went for a drive and I really like the new shocks. The ride is much nicer without the harsh jolt when I hit a bump. I'd say it's as nice as a C3 can be with stock suspension. The new spring cushions had to make some difference with the old ones being all squished out flat.

I'm not pleased with the brakes though. I put everything together last fall and tried to bleed the brakes. Unlike the first set of rear calipers I got that bled out cleanly right away with the wife pumping the pedal, the second set just seemed to keep discharging air bubbles to no end. So I decided to just forget about it for awhile and buy a power bleeder over the winter. I got a Motive Products Power Bleeder from Amazon who had the best price. Online reviews seemed to like it except that it could leak pressure out of the ends of the cover plate that goes over the top of the master cylinder if you tightened the J bolts too tight. It seems that the one size fits all cover plate is enough wider than the Corvette master cylinder that when you tighten down the J bolts on the outer sides it will bow the plate up a little in the middle and won't hold 15psi pressure (or 10). Last week I couldn't get it to hold pressure no matter how I tightened the bolts. It was either not enough or too much. Well the online reviews also had a fix. 2 6" C clamps. So a quick trip off to Harbor Freight netted me a pair of 6'' C clamps at a reasonable price with the 20% off super coupon. I clamped the cover back on with the master cylinder topped off and lightly snugged the clamps. It had no trouble pumping up 15 psi in the tank and holding it. Fantastic! One man break bleeding.

I went to the right rear and opened the outer then the inner bleeders and then repeated again. I got some air bubbles out on the first opening on each bleeder but none after that. I left each one open for about 5 seconds each time. Then I went over to the left side and did the same. Topped off the mc then did it again. I never got any air other than the first time I opened them. Just clear clean brake fluid in the clear tube to the bottle. Topped off the mc again and buttoned it up.

Got in it to go for a drive and I was immediately dissappointed in the low brake pedal. It was firm but low. Not on the floor but I expected more with the new pads. It wasn't any better that before. My driveway is stone so I did a couple test stops from just off idle to see if they were working and they were sliding in the stone so down the road I went to the corner then I turned down another county road with no traffic on it most of the time. I followed the bed in proccedure to the letter. 5 stops from 45 down to 10 in rapid succession then 5 more from 35 down to 5 all without allowing the brakes to cool. Then they called for 5 minnutes of driving without stopping or holding the brake pedal down to cool the brakes and set the pads. By the time I got home I had 15 minutes in without braking except for turning in the drive. The front brakes were warmer than the rear but it seemed like it was pulling to the left when I did the bed in. I took it back out and drove for a half hour normally. I stopped from 50 a couple times and got out and checked temperature at the rear wheels and front. The front were always warmer but they should be. The rears were the same side to side even though on a firm stop it would be easy to squeel the left rear. I think that the left rear over-braking is causing the pull to the left. It isn't locking up, just braking harder.

I know the discs were clean and also the pads were clean before I put things back together. I don't understand why I can easily lock up the left rear and why the pedal is so low. The pads are Bendix ceramic metallic. Supposed to be modern OEM type. Maybe pulling the pads and inspecting them or even swapping them around would help the pulling but I wish I had a better pedal. I could try bleeding again.

Any ideas?

Some questions...

Were the front calipers also replaced or rebuilt?
Is the master original?
When you bled the back, did you raise the back end up high during the bleeding process?
During bleeding when the bleeders are open, did you tap on the caliper housing with plastic hammer?

I tested the Motive bleeder and found it not acceptable. Too many leaks. Sent it back. The best, small tank pressure bleeders are the ones GearWrench and a few others sell. Unfortunately, they are not low priced as is the Motive.

The GearWrech 2901D is the one I have in my shop.
  • Thread starter
  • Moderator
  • #17
Thanks for the reply.

The front calipers were replaced a few years ago, also sleeved. I haven't had any problems since with them. I also had no problem bleeding them with the brake pedal. Fluid in the front half of the MC is clear also. I was thinking that while I was at it that it wouldn't hurt to push some fresh fluid through them just to verify that there is no air in them and that no crud is in them.

The MC is original. I don't think there is any problem with it though. I can hold pressure on it for as long as I want and it holds with no bleed down.

Yes, the rear was raised up to where the MC was level.

No, I didn't tap on the caliper with a plastic hammer. I will the next time I bleed them.

I can see your point with the Motive bleeder. It should not leak when used on a MC that it is sold to service but once I secured the MC cover with the C clamps it was completely leak free. I could pump it up to 15psi and it held. You shouldn't have to buy more tools to make the one you bought work though.

On the "low pedal", does the height change any if you pump the brakes?
  • Thread starter
  • Moderator
  • #19
Yes. It will pump up about an inch. There must be air in there somewhere.

You may have a problem with "piston knock-back" due to rotor run-out and the run-out may not be the rotors themselves, it maybe because your rear bearings are worn.

Look over in the featured articles section for the BBfH series. In that you'll find a couple of articles about C2/C3 disc brake diagnosis and service.

Corvette Forums

Not a member of the Corvette Action Center?  Join now!  It's free!

Help support the Corvette Action Center!

Supporting Vendors


MacMulkin Chevrolet - The Second Largest Corvette Dealer in the Country!

Advertise with the Corvette Action Center!

Double Your Chances!

Our Partners

Top Bottom