Finishing up my 76, the last mechanical thing to do is to get a good brake pedal. The PO said he "re-did all the brakes". It looks like all the calipers, pads,parking brake shoes and hardware and some brake lines were replaced. I just repaired the parking brake by freeing up the actuating lever and replacing the cable and it works great. The rear calipers were off for this repair. Since I've had the car, the pedal has always been low and soft. After the parking brake repair, I bled all the calipers, LR,RR,LF,RF.
Pedal better but not right. Replaced master with rebuilt, bench bleed and rebled the calipers. Pedal now is pretty good for normal driving, but panic braking is no good. The brakes will not lock up. The power booster and check valve appears to be working properly. I was thinking the pads are either glazed or contaminated by brake fluid? Can the booster still be bad after passing the "pedal drop and pump" tests? I used gravity, pump and even a pressure bleeder to bleed the lines. I dont suspect air bubbles.
Any ideas??

 

I suspect the master clyinder. Rebuilts are always a crap shoot.

Always buy a new master clyinder.

And yes the booster can still be bad after the pump and drop test.

One question. if you pump the brakes does the pedal come up?
If so I'd bet on there still being air in the lines.

 

No, the pedal doesnt pump up. I did have that problem prior to the work I did. I bled the *%@# out of them brakes...

 

Doesn't sound like the booster. I suppose you could have the wrong diameter M/C giving you to little pressure for a given pedal force. Yes, the pads could be glazed over. There is a procedure to burnish them in, but I don't recall what it was, maybe do a search or someone with this knowledge will chime in. It's easy and cheap to replace the front lining if you suspect those.

 

I've got the same problem with my '68. Good initial braking force, but at low speed, increasing the pedal pressure doesn't result in more breaking force. There's no air in the lines (won't pump up), and the power booster seems to be working. I replace the M/C as well, with no change. One thing that did effect the braking action, was lengthening the actuator rod that pushes the M/C diaphram. It raised the pedal actuation point, but didn't change the low speed stopping problem. I haven't replaced the pads, but will try that next. Good luck.

Jimbo

 

JImbo,
Seems like we have a similar problem...My pedal is rock hard on application, but doesnt seem to get a good "bite"; yours?

 

Yeah, my pedal is rock hard at about 1/2 depression. I'm going to swap some pads this afternoon and clean the rotors, to see if that makes a difference.

Jimbo

 

I can't confirm this potential remedy as yet since I just learned of it while on a trip (with my Vette) driving through Canada from Alaska last week. I, too, have had nothing better than spongy brakes since I've owned my '76 back in 2000. The original owner said he had a mechanic bleed the system intensely more than once but each time it failed to improve the braking action more than just a minor bit (but it soon went back to the original spongy feeling rather rapidly). He also bought new calipers and pads, lines, etc., throughout and still no better effect could be produced.

Anyhow, while getting a tire repaired along my trip I encountered a fella who was a previous old vette owner several times over and has restored several 70 era vettes from frame up. I mentioned the brake action I was experiencing and asked if it was typical of the year and he instantly informed me that the problem was in the wheel hub/rotor placement on the axle, itself, and nothing to do with the brakes. He pointed out that my brake calipers looked rather new, pads all in great shape then went on to inform me that Corvette rivoted the wheel drum/rotors in place on the axle before turning the rotors to ensure exact alignment and balance. He said it was obvious the rotor rivots had been ground out to remove them from the axle and he said if they are not marked and replaced in their "exact" location where they were originally that even so much as a .001 out of alignment they will microscopically wobble and cause the calipers to pump in air in behind the pistons while performing braking actions. You can bleed the system to death, he said, and it'll only improve the performance for the first braking actions at the onset of driving off, but will return to the spongy effect within just a few miles of use.

He said to acquire a hub alignment tool of some sort (I'm a ditz at remembering names of things but know what it is he was refering to) and said to re-remove the rotors and reposition them with proper ensured alignment rather than just back on the hub aligned with the bolt holes as anyone would suspect. Once that is done with percision, then re-bleed the system again and he said the brakes will return to be sharp and quick to grab as one would expect. It sounded reasonably logical to me so I'm going to give that a try this winter since I've had it assessed by every mechanic I know and at best they say what they offered was all they could do to improve the braking action. I know for a fact that can't be all there is to improving them and if what this Canadian fella said is true then it's not as much of a headache as I thought it might turn out to be. If anyone beats me to doing it, please post the results for the rest of us to see and review. Thanks.

 

I believe what he was referring to is rotor runout. You need a dial indicator and some shim stock. There was a good article in last months Corvette Fever (I think) on how to do it. Not that complicated.

 

Well, I didn't replace the pads, but I did scuff them up with a wire wheel on a grinder. When I took them off, they did look glazed and I thought I might have a Eureka experience. However, after scuffing the pads and cleaning the rotors, I didn't improve the braking action at all.

The thing that baffles me, is that the first application give me the most braking force all the way to the stop. If I let off and reapply the brakes, the braking force is less than the first application. On a third application at slow speed, I have to look for an escape lane, as the pedal is firm but there is little stopping force. This doesn't act like any air in the lines from run-out or residual.

I have another master cylinder that works well on my '73 I'm going to try. But I'm just about at wit's end. Any other suggestions?

Jimbo

 

Jimbo,
I sent an email to a forum member who was referenced in an earlier thread. He sent me a long, helpful email. I havent tried any of his suggestions yet, but I intend to get back to work on the car this weekend. Your problem sounds exactly like mine. Keep us posted.
I'll do the same

 

If both front and rear brakes aren't working, that points to the booster since it's the only common component. Disconnect the vacuum line from manifold to booster (plug manifold) then carefully make an easy stop. Does it feel the same as at the end of your 3 stops? If so, must be the booster.

 

What type of brake pads?

Performance of different pads will vary dramaticallly. Some will stop stopping as they get hotter. Some are horrible cold.

Good quality pads will make a night and day difference.

I have "new looking" NAPA pads which are unknown vintage/quality that came on the car when I bought it. I replaced EVERYTHING in my brake system that could make a difference, except these pads. Pedal is rock hard (a good thing). Initial bite is poor and gets worse as the pads heat up from multiple frequent braking. I know it is the pads. Too cheap/lazy to replace them at the moment though.

I will most likely install HAWK or EBC green stuff pads, when I get a chance. These made a huge difference over street pads on my other vehicles.