New C3 Caliper Bleeder Screws Leaking

[Jack O.]

As part of my trailing arm rebuild project I painted my calipers and replaced the bleeder screws and plugs. Although I used gravity bleeding before I decided to try it with a mityvac this time. So in trying to follow the mityvac procedure I've found that I cannot get the mityvac to hold vacuum unless I replace the new bleeder screw (stainless steel purchased from Corvette Central) with the old bleeder screw. When I use the new bleeder screw the vacuum leaks down very quickly. The two look basically the same but it appears the new ones are not fully seating and I've tried snugging them up tighter but to no avail.

Anyone had a similar issue? Just curious, most likely I'm going to simply punt and put the old ones back in and be done with it.

I would had preferred to not use SS but most places I checked only had SS.

Thanks,

Jack

[Steve G.]

Re: New C3 Caliper Bleeder Screws Leaking

As part of my trailing arm rebuild project I painted my calipers and replaced the bleeder screws and plugs. Although I used gravity bleeding before I decided to try it with a mityvac this time. So in trying to follow the mityvac procedure I've found that I cannot get the mityvac to hold vacuum unless I replace the new bleeder screw (stainless steel purchased from Corvette Central) with the old bleeder screw. When I use the new bleeder screw the vacuum leaks down very quickly. The two look basically the same but it appears the new ones are not fully seating and I've tried snugging them up tighter but to no avail.

Anyone had a similar issue? Just curious, most likely I'm going to simply punt and put the old ones back in and be done with it.

I would had preferred to not use SS but most places I checked only had SS.

Thanks,

Jack

Haven't had the issue with a ss bleeder screw, but it is a problem with ss lines for the same reason.

SS is very hard. The seal between the line and body or screw and body is metal to metal, no soft gasket material between. So it relies on the two metal parts to take the shape of each other to form the seal. You have to tighten a ss line a lot tighter than a soft steel line (or screw) to get the deforming action to take place.

Steve

[Jack O.]

Re: New C3 Caliper Bleeder Screws Leaking

Another question, are the bleeder screws supposed to be silver or gold colored plated? How about the plugs?
Thanks.

[Jim T.]

Re: New C3 Caliper Bleeder Screws Leaking

Don't know if original type bleeder screws are still available from a GM parts counter.

[Alan S.]

Re: New C3 Caliper Bleeder Screws Leaking

Hi Jack,
The information I have (TIM&JG) indicates the original bleeder screws were zinc plated.
I don't see any indication of a dichromate wash being on them.
I'm not sure about the plugs, but I'd think they'd be zinc plated too.
Regards,
Alan

[Wayne W.]

Re: New C3 Caliper Bleeder Screws Leaking

They have a brass fitting in the caliper. If the SS are shaped different they probably are not sealing because the old fittings have made their impression in the brass fitting.

[Gene M.]

Re: New C3 Caliper Bleeder Screws Leaking

Jack,
Some of the "foreign" sourced have the taper seat point machined incorrectly. A high magnification shows this as compared to an Original GM part. When an impression is there as Wayne explains to insert a fitting of differing taper causes the issue you are experiencing. I find the stainless fittings work just as well provided the taper is machined correct.

It is sad one needs to inspect reproduction parts under a microscope to insure they were made correctly.

Just as a side note when using 5/6" screws be aware there are pieces out there that are metric thread that will almost screw together but they mess up the thread.

[Jack O.]

Re: New C3 Caliper Bleeder Screws Leaking

Thanks everyone - believe you hit the nail on the head regarding the leakage as the tips of the new screws are slightly different than the old ones .
I'm going to check GM parts - sometimes I blow right pass that option without really considering it.
Jack

[Gene M.]

Re: New C3 Caliper Bleeder Screws Leaking

Another question, are the bleeder screws supposed to be silver or gold colored plated? How about the plugs?
Thanks.

Jack, I think you will find the originals were zinc plated, a silver color appearance.

[Joe L.]

Re: New C3 Caliper Bleeder Screws Leaking

Thanks everyone - believe you hit the nail on the head regarding the leakage as the tips of the new screws are slightly different than the old ones .
I'm going to check GM parts - sometimes I blow right pass that option without really considering it.
Jack

Jack------


The last available GM bleeder valves for your application were GM #12489575. They are now discontinued. Dorman offers an aftermarket replacement but I do not know what the manufacturing source is.

[Jack O.]

Re: New C3 Caliper Bleeder Screws Leaking

Thanks Joe. Hampton Chevrolet didn't have any but gave me a short list of Chevrolet dealers that still have some in stock. I picked up a couple from NAPA (actually made in the USA no less) and if they do not work I may reach out to one to those dealers.

[Joe L.]

Re: New C3 Caliper Bleeder Screws Leaking

Thanks Joe. Hampton Chevrolet didn't have any but gave me a short list of Chevrolet dealers that still have some in stock. I picked up a couple from NAPA (actually made in the USA no less) and if they do not work I may reach out to one to those dealers.

Jack------


I'm the kind of guy that ABSOLUTELY LOVES stainless steel. I like stainless steel more than gold or silver. I'd much rather wear a stainless steel watch than a gold watch. However, I don't like stainless steel for brake lines, fuel lines, or brake bleeder valves.

[Jack O.]

Re: New C3 Caliper Bleeder Screws Leaking

No luck with the NAPA bleeders either. Seems odd but if it really is that the caliper has been formed to the bleeder then I may have to make sure I put all the bleeders back into the specific caliper hole I pulled them out of, correct? I know its not the mityvac itself leaking because I tested it to ensure it holds when plugged up. What a pain. Only other possibility is it's leaking around where the hose is attached to the bleeder but that doesn't seem likely. Seems like it will surely leak brake fluid if it doesn't hold vacuum.

[Richard M.]

Re: New C3 Caliper Bleeder Screws Leaking

Jack, If I understand correctly, you're using the Mityvac properly, but the bleeders are not designed for that reverse operation. In order to use the Mityvac you must seal the threads on the bleeders to prevent air escaping and getting sucked into the Mityvac via the bleeder threads.

If you're leaking after tightening the bleeders into the calipers, then I have misunderstood and yes it appears your bleeders are wrong. If not.....

Remove the bleeders and wrap several layers of teflon tape around them. You will need a wrench to get them back into the calipers when taped. Reinsert them so the tip is just off the flare in the caliper so you get flow. Then the Mityvac can do it's job. But keep in mind that tool is not the best way to get a final bleed.

I have found after a Mityvac exercise(with teflon taped bleeders), I then let them gravity bleed. For rears I use a Tee hose to get both halves going at once. I use DOT 5 Silicone fluids exclusively these days on complete brake rebuilds, and gravity always works best to minimize micro-bubbles which can yield a soft-pedal later. Most times after a good 1 hour or so gravity bleed, I get a perfect pedal!

Rich

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[Jack O.]

Re: New C3 Caliper Bleeder Screws Leaking

Rich,
Thanks for the detailed explanation and yes, this is an issue when my bleeders are tightened.

Aft thinking about it a bit more though I believe this could still be an issue caused by leakage around the threads which then could be resolved, or at least partially addressed by the thread tape or grease approach.

Even if the bleeder is properly sealing against the caliper vacuum could still pull air from around the threads through the hole in the bleeder screw, right? I believe so and hope so since I can easily address this as opposed to dealing with leaky calipers.

Jack