65 Disk Brake Woes

[Daniel K.]

[Joe L.]

Re: 65 Disk Brake Woes

Dan-----

Did you replace the rear rubber brake lines? If not, that could be a problem. Also, check them to be sure that there is no "kinking" and check the steel brake lines to the rear for any evidence of kinking or damage.

If none, I'd say that there is a problem with the calipers. Something is not allowing the pressure to "bleed off" when the brake pressure is released.

[Daniel K.]

Re: 65 Disk Brake Woes

Thanks Joe. All rubber brake lines were replaced as well as the rear metal lines. I inspected all metal lines for any evidence of ding, bends or kinks.
Also, even with the brake pedal feeling solid, braking performance is dismal. Thanks again for responding.

[Kevin Whiteley]

Re: 65 Disk Brake Woes

Dan,

You indicate that the problem is only with the left/right rear brakes. I know this may sound simple, but is your emergency brake released or the wire possibly kinked/caught at some point. This may be why only the rears are smoking.

Pad thickness shouldn't be an issue since the brakes are supposed to operate nominally with different thicknesses due to wear and/or replacement. If it's not the emergency brake somehow, Joe is probably right, that the pressure is not bleeding off after your braking effort is released.

[Brian Monticello]

Re: 65 Disk Brake Woes

Dan,
With the pads off can you push on each of the four caliper pistons on each caliper with ease? THey should sile in and out easily with some tension caused by the spring and mass on the brake fluid in the line.

If the pistons check out I'd look to your MC (assuming you have a dual). I've never seen the guts but ther has to be some way for the pressure to release back into the MC. If this passage is blocked or restricted you'll experience the symptoms you explain.

BTW, don't drive too much in that condition. I had a sticking caliper that I put off replacing on a driver - relax not a corvette (got so bad that the caliper get red hot and the fluid boiled). The rotor is warped, the pad squeeqs like hell, and I worry about the wheel bearings. I wouldn't want to worry about rear bearings on a midyear.

good luck
Brian

[Daniel K.]

Re: 65 Disk Brake Woes

Thanks for responding Kevin. I too suspected the e-brakes on the first go round. I am lucky to have a lift in my shop so I can quickly look at these issues and determined that the e-brakes were not the cause. I too am beginning to belive the calipers are suspect. Thanks for your help.

[Daniel K.]

Re: 65 Disk Brake Woes

Thanks for responding Brian. During the brake pad removal process and sanding, I attempted to press on the pistons....rock solid....but, not having experience with disk brakes, I thought perhaps this is normal. Now I know this is not a usual condition. I won't drive the car until these issues are resolved. Thanks again for your help.

[Brian Monticello]

Re: 65 Disk Brake Woes

Dave,
Just though of something else. In order to isolate the caliper pistons you need to make sure that any residual pressure is relieved and is not due to the MC. I'd open both bleeder valves on each rear and then try and push the pistons in - assuming they're "out" (towards the rotor) to begin with. Fluid may shoot everywhere if there is pressure so protect your paint.

If the MC causing the pressure than it may cause the caliper pistons to appear to be stuck.

Think of this too (I learned this the hard way) - heat causes things to expand so a brake caliper may work perfectly when cold and start to bind up at normal disc brake operating temperatures. Added pressure causes more friction, then more heat and expansion and binding, then more friction... and next thing you know you're smoking. After things cool off it may appear to function perfectly again.

Brian

[Daniel K.]

Re: 65 Disk Brake Woes

Thanks again Brian. I'll try your suggestion this afternoon. I have a kit to catch all the fluid during the bleeding process so I'll use this to bleed off the pressure. I am very concerned about the paint on this car as it was repainted in 1979 and is in near perfect condition. Thanks again for your help and advice.

[Joe L.]

Re: 65 Disk Brake Woes

Dan-----

You will need to open the bleeder valves if you want to press the pistons in easily. However, they can be pressed in without doing this, but you'll need to get a lot of leverage on the piston to do it. You certainly won't be able to do it by pressing in with your fingers, as you could if the calipers were uninstalled.

One other thought here that I was going to make earlier, but didn't since it seems that you had some sort of problem with these calipers from the beginning (i.e. the non-contact of the rear pistons with the pads even after you had bled the system). Anyway, my thought is this: check that you have the CORRECT master cylinder. Master cylinders for Corvettes with disc brakes have NO check valve in either the front or rear circuit. However, cars with drum brakes DO have such check valves in both circuits. Most cars with hybrid brake systems (i.e. front disc, rear drum), have no check valve in the front disc system, but DO have a check valve for the rear drum system. Perhaps, you somehow got one of these. The check valves are internal to the master cylinder (usually, in the inner part of the outlet ports), so you can't easily see them. However, the first thing to do is to confirm that the PART NUMBER for the master cylinder you got (likely found only on the box that it came in) is the correct one for your application. If it's the correct part number, then you may have to check the actual master cylinder for the check valve(s) since it could still have been misboxed or "switched" prior to your receiving it.

[Daniel K.]

Re: 65 Disk Brake Woes

Thank you Joe.

I'll see if I can determine if the MBC is correct. On the exterior, it looks identical to the one I removed from the car and on my invoice it does not reference a part number but simply indicates "NEW, Master cylinder (1965 non-power). I have removed the calipers from the car and attempted to move the pistons....I did not press too hard, however, they were....rock solid. I did not attempt to open a bleeder valve and thought the connection from the fluid system would suffice for an "open" environment. I am sending the units back to the vendor this PM and I'll ask about the MBC as well. Thanks again for taking the time to help out. Your advice is appreciated. Dan K.

[Joe L.]

Re: 65 Disk Brake Woes

Dan------

For some strange reason, I was thinking that your car was a 67 (that's why it's strange since the thread title clearly says 65). All 67+s (and 65-66 with power brakes) use a dual circuit master cylinder. However, your 65 with standard brakes uses a single circuit master cylinder. So, it's much more unlikely that you have a master cylinder problem. If you did, you should have problems with the front as well as the rear brakes.

[Daniel K.]

Re: 65 Disk Brake Woes

Thanks Joe.

Yes, just the rear brakes are the problem......thus far.... Anyway, I did e-mail the vendor as to how to ID the MBC they sent to me and invited them to look at this discussion board. Perhaps they will post a response....

Thanks again for your help.

Dan K.

[Dave McDufford]

Re: 65 Disk Brake Woes

I suggest going back to Joe Lucia's original response to check for kinking or damage to the only steel brake line that is common for both back brakes only. That is one between the junction fitting right below the master cylinder and the junction fitting near the left rear frame kick up. The likelyhood that two calipers are damaged and acting the same is less than something being wrong with that line. Also, if it were a master cylinder problem it would also affect the front brakes.
The pressure of appying the brakes can get the fluid past the objection. However, it could be just enough blockage to allow partial but not total release of the pressure.
You indicated you replaced the rear steel lines but was this line (which is hard to get to with the body on) replaced?
As mentioned earlier and you did not do, I would see if you can push the calipers back with the bleed valves open. This will tell if it is a residual pressure problem.
My 2 cents.
Dave

[Daniel K.]

Re: 65 Disk Brake Woes

Thanks Dave for your input on this problem. I did look at the line from the junction where the line splits off and goes to the L/R front and then the line to the rear brakes. As you indicate, it is nearly impossible to see that line all the way back as it does run along side of if not on top of the frame, and with the body on, it appears to be well hidden. This car is also fitted with side pipes. Your suggestion does make sense that the probability of "both" calipers being bad is perhaps unreasonable and does put a new spin on things. As I removed the calipers today, I noticed that the brake pads were not under pressure at all, but appeared as if they were in a more normal position (just a little bit loose against the rotor). In reference to the calipers and the bleeders, do the bleeders in fact need to be opened even if the calipers are no longer in a closed system? If so, I'll certainly open the bleeders and press on the pistons to see if they do move freely. Overall, the brakes are deplorable in operation. My 63 stops on a dime, and this 65 acts as if it's just thinking about stopping..... I appreciate you comments. I'm thinking you are right about that long line to the rear brakes. Dan K.