"Aftermarket new calipers"? Exactly what did you install? Do they have two bleed screws like stock calipers?
Concerning the master, jack up the rear of the car and slowly press the brake pedal while watching the fluid in the master for air bubbles. Raising the rear of the master higher than the front should cause air bubbles to travel back to the piston seals. When you step on the pedal the bubbles should exit through the holes in the bottom of the reservoirs.

 

I installed these:

 

Did you bleed 4 bleed screws in the back?
No mention of those front calipers having stainless sleeves.

 

It's actually 3 bleeders in each rear caliper, not 4. Yes I bled them exhaustively.
They are not stainless sleeved, but I think that is only an issue of longevity - it to keep the bores from rusting if your brake fluid absorbs water. Flushing the brake fluid annually is said to prevent any problem though.
I debated whether to buy new repro or remanufactured. 2 schools of thought. New is new, but China built and potentially not to exact specs (who knows). Remanufactured units are stock quality but the rebuild quality is not always good. I went with new, but maybe the wrong choice?

 

A stock Corvette rear caliper has two bleeders on the top of each caliper half. On the bottom is a plug not a bleeder. If you opened the bottom you let air in.

 

fwillison
I'll go look at my April 1968 just for ***** & grins for comparison purposes but I have a question:

DId you ever put the original good master cylinder back in to test it? Meanwhile I'll go get a picture of my 68 brake lines.

Pressure bleeding has always worked fantastically for me. I even do my other cars with it and it's great for system flushing.

I'll be back....

OK..... Front goes FORWARD and the rear goes AFT... on my 68

 

I'll attach a pic of the rear calipers I put on. Sure looks like 3 bleeder screws with rubber caps. Same as the old ones I removed. See what you think.



.

 

Thanks for the pic. That comports with the AIM diagram as well. I guess a previous owner switched the lines. I have them corrected now, though, so front goes to front and rear goes to rear. As far as the original master cylinder, when I went to bench bleed it prior to putting it back on the car it fell out of the vice and broke the mounting flange. My luck. I was pretty frustrated and got sloppy, which is never a good thing when working. It's probably just as well though, since when I removed the dust boot to bench bleed it, there was some seepage noted so it probably had a slow seep from the seals and was getting ready to give up the ghost anyway.

 

I'm going to assume the end with the two bleed screws is pointing up or should be. The end with a single screw on a factory caliper is a plug and is Not opened when bleeding brakes. Look online at a AC-Delco original style rear caliper.

 

That makes sense since I have never gotten any air out of the bleeder they put in the plug hole - just fluid. Since it is on the bottom - duh. I thought it didn't make sense. But the rear seems ok, it's the front that is the problem.
And yes the end with the two bleed screws is pointing up.

 

but you state with the rear hoses clamped the pedal hits the floor pointing back to the master.
did you compare the distance from the master mounting surface to the piston cup on the original and the Dorman?

 

If I clamp the front hoses the brake pedal gets solid. Nothing changes when I clamp the rear hoses. To me that indicates a problem distal to the front brake hoses, since clamping there fixes the problem with the hydraulics. That would indicate the front calipers are bad or there is air in them. At least to me.Think of it like this - if there was an obvious leak in a front brake caliper the pedal would go to the floor. Clamping the front brake hose to that caliper would give a solid pedal again. Clamping a rear brake hose would not change anything. That's my situation. I'm going to rebleed the fronts tomorrow. I have made a little progress as noted before. I get some pedal now, it no longer goes to the floor. But spongy.

 

Two things come to mind:

First-There are two master cylinders with two depth of pushrod cups. The pushrod should fit with almost no slack. You need to check this with any new corvette master. You need this gauge or a digital caliper to check the pushrod depth.

Second - did you bleed the MC on the bench and tilt it both fr/bk while doing so? That ensures all the bubbles get out.

 

Great post, I've not heard this anywhere before. I was just thinking about the pushrod to MC specs last night while ruminating about my issue. I noticed when installing both master cylinders (which were identical Dorman units) that the MC was held off the firewall a quarter inch or so and it drew up tight to the firewall as I tightened the nuts. I didn't think too much of it at the time, but lately beginning to wonder if this indicates a problem with pushrod adjustment (or now as you point out - master cylinder cup depth).
Is it possible that the pushrod or MC cup is being inordinately "preloaded" preventing the cup from fully backing out on release of the brake pedal? If so what would this do? Maybe reduce the "stroke volume" of the master cylinder? One point on that issue is the fact that reversing the reservoirs to properly route the front to the front and rear to the rear did not change the location of my issue (reflected by the hose clamping results given previously).
Where do I get that special tool? What are the specs I'm measuring for?
Also, I've never heard of 2 different master cylinders for the car - can you elaborate?
Thanks!

Regarding bench bleeding, I did carefully bench bleed the cylinder but did not tilt it while doing so. Hadn't heard that before. In addition, there was some slight leakage from the MC when installing since it is impossible to get the hard lines in the cylinder without losing a little bit of fluid while doing so. This would be a universal issue, but never heard anyone mention the ramifications of it. I wonder.

 

The two are power and non- power, different diameter bores, 1" and 1.25". I mentioned to you in a previous response to compare the Dorman to the original in the area of the mounting surface to the piston.

 

The problem: Drivestar Calipers
Dorman Master.

Solution: Delco everywhere.

 

Could be. But they do sell a lot of these and most people seem to be satisfied. My thought was that everything is now made in China, and different brands are often from the same factory in China with a different label stuck on. But AC Delco could be different.
. The name

 

Here is a to the tool on Amazon.

Here is a link to a previous discussion with a pic showing the two different "cup" depths.

I have heard some of the inexpensive suppliers are mixing these up.

Apparently the seals can even get damaged while manually bleeding the brakes? Learn something new every day.

 

That is a very rare part. I sold one of these that was NOS along with the caliper brackets to a restorer about 20 years ago.

 

I know someone recommeded the vacuum bleeder, I would not. I was not a fan of anything except the old way of doing it until I saw how GM did it and by accident a new company started making a small pressure bleeder. They sent me one in like 03 or 04 and we used it on the race can and I could not believe how easy and how awesome it was. It was a Motive bleeder and you can find them anywhere, it is a life safer on bleeding the older Vettes as well and it is the only way I would do it now.