It's been too long now and I can't get the master cylinder to pump clear fluid back to the rear brakes, always with bubbles. Took the advice to bench bleed, twice actually, and still no luck. Pulled the guts out and inspected seals and bore under magnifier, no sign of any problems. Brand new M/C by the way.

Now I've taken the guts and replaced them entirely, still same symptom. What I fail to understand is how the rear chamber actually pumps any fluid out! In both bowls, there's two holes into the bore, one small and one large.

In the rear bowl, the large hole feeds the area of the piston between the first and second cup seal - why? How does fluid get to in front of the second cup seal where it can be pushed into the rear brake line? Is that what the small hole is for? I've been told that it's drawn over the second cup seal on the back stroke. That doesn't make much sense to me tho, but nothing does right now. The small hole appears to be in front of the second seal, but why so small? And why two holes?

And where is the air getting in the system anyway? There are no visible leaks, but obviously on the return stroke, air is being sucked in somewhere. I have new flex lines as well, but everything else is as it was when I parked it.

And by the way, no problem bleeding the fronts, which brings up another point...when bench bleeding the M/C, I get quite a forceful squirt out of the front fitting, but next to nothing out the rear. This was with either set of internals.

Any and all suggestions appreciated, this has gone on long enough. And yes, I know that I could bleed them with a power bleeder, but I still think something is not functioning correctly, I shouldn't be getting air into the rear lines regardless.

 

If you are sure that things are right in the master cylinder then there is one simple thing that I do to remove the air. Elevate the rear of the car as high as you can so the master cylinder as level as possible and bleed the right rear first and then the left rear. Good luck
Billy

 

No, I'm not at all sure things are right in the M/C, altho I've compared this new one to another and found no difference. But I don't need to elevate the car, the M/C has been bench bled. And I always start at the right rear, that's not the issue. It appears to me the M/C is drawing air from somewhere into the chamber for the rear brakes rather than filling with fluid, altho this only happens once installed in the car.

UPDATE: After reading this, I realized that my questions about the functioning of the M/C aside, I may have answered my own question. If the M/C pumps clear fluid (no bubbles) on the bench, then it's obviously pulling air thru the brake lines when connected in the car. And I may not have any visible leaks because it may not be able to create sufficient pressure with all the air in the line. Anyway, something for me to investigate further.

 

According to the Corvette manuel you are suppose to start bleeding at the left rear first than the right rear, right front and then left front. Its a different setup then most other cars where you would start at the right rear. This could be your problem. I have heard over the years from plenty that this was a major cause of still having air in the system from doing the procedure wrong.

Question when you bench bled did you do it with a hose kit pumping it back into the mast cyl? I find this to be the best way. Some just close off the side ports with plastic fittings or the use of their fingers. Also another question are you doing a power brake or manuel brake car? Did you do any adjustment to the rod behind the master cylinder coming out of the fire wall or power booster? This could mess things up if adjusted wrong. Do you have the correct master cyl. for your car? Power and standard are different? You could also have a leak at one of the rear calipers and not notice it because it is so minor. Have you done a good visual on them? I had a problem with one of the rear calipers one time where I found a waxy build up around two of the four pistons under the top seal on one side of a rear caliper that didn't leak fluid out to where it would drip on the ground but when compressed caused air to suck into the caliper. I had to split the caliper and do a rebuild with new seals although this caliper had SS sleaves and just needed a good cleaning out. I used brakecleener and a 3m scuffing pad to clean it up well before installing new seals to it again. I hope some of this info helps. Corvette brakes when bleeding can be a headache sometimes.

 

I've actually tried it both ways with no difference.

Yes.
No, but please explain further. The booster rod seems to fit the M/C correctly, no discernable free play and no preloading.

Booster came from a '66 or '67, M/C is for a '67 w/power brakes.

No, this I have not done. I guess since I was changing over to the new booster and M/C, I was presuming my problems arose there.

No kidding!

I guess I'll go have a look at the calipers and the connections to the new flex lines in the rear. I'm about out of ideas at this time. I think I might try to get a pressure bleeder to see if that will uncover any leaks.

 

I had the same problem on a 1967 with PB, fought with it for hours.The fronts world bleed out fine then the rears would get bubbles. Drove me nuts, Everything was new and nice. I borrowed a pressure ball and the problem was solved. Good luck

 

I am not sure of your bleeding process. Most people will pump the pedal repeatedly with the bleeder closed, hold the pedal down, open the bleeder, close the bleeder and pump the pedal repeatedly again. My experience is this works ok when the master cylinder and the brake lines are full. For say a caliper replacement. With a new master cylinder and the lines have drained I use a "single stroke" method to start off with. Bench bleed the master, push brake pedal down slowly. As the pedal is going down open the bleeder at a rear wheel. Before the pedal is fully down close the bleeder. Let the pedal up slowly. Repeat process 10-12 times and then check pedal resistance with bleeder closed (Pump the pedal). When you get some brake activation and just want to finish up then use the "multiple pump" method.

Works for me.
Tom

 

Please explain a 'pressure ball'? Is this just another form of a pressure bleeder. And weren't you still concerned that air still had a pathway into the system?
I've tried every possible approach to manual bleeding up to this point. I've always preferred the single stroke method and have never experienced this before.

UPDATE: I have disconnected the rear caliper line and have bubbles before the caliper, so still searching...

 

'67 was the only year that manual and power brakes used the same exact master cylinder (casting #5455509, stamped "DC" at the front), and it had the "deep" hole in the rear piston, with a "long" booster rod; power brake master cylinders normally have a "shallow" hole in the rear piston and the "short" booster rod. I'd check the protruding length of the booster rod past the front surface of the booster vs. the depth of the hole in your rear piston and make sure they're compatible.

If that checks out OK and the internal stack in the master cylinder is correct, try a pressure bleeder (www.motiveproducts.com); takes all the grief out of bleeding Corvette brakes for about sixty bucks.

 

You didn't say if you had two bleeders on the rear calipers or three. THey came with two but I've seen rebuilders putting one on the lower outboard side. According to Stainless Steel Brake Corp (SSB) the proper way to bleed a system is to go to the bleeder furthest from the MC. That would be the right rear outboard bleeder. If you have two outboard bleeders then do the top one first then the bottom. Once you start to get a good stream out of that bleeder, go on to the next furthest (right rear inboard). Get all three producing a good flow (sometimes using a soft mallet on the calliper while bleeding helps dislodge the air) go the left outboard bleeder, then left inboard and repeat the drill. One other thing to do is to raise the rear of the car so that from the floor to the center of the wheel flange is 15". This puts the vehicle in the best position to the MC for bleeding.
Another thing to mention here is to make sure that the end of the bleeder hose MUST be submerged in brake fluid so that air isn't sucked back into the system.
I've done this many times on my '66 and always get a good bleed.
Another thing to consider is that you just may be sucking air through the bleeder threads that gives the appearance of air in the lines.

Good luck.....

 

Went to the store for a pressure bleeder and they didn't have the correct adapter for this style master cylinder. But after considering the problem, I just fashioned my own with a piece of 1/2" aluminum sheared to a rectangular shape that covered the top of the M/C. Drill and tapped a hole for the air fitting, cut a cork gasket to match, and C-clamped it to the M/C.

As soon as I hooked up the air, the problem was apparent, a tiny hole in the rear brake line about 10" down from the M/C. Previously, any leaking fluid apparently ran down the line and onto the top of the frame, puddling somewhere out of sight. With 15PSI of pressure applied, the leak was easy to see.

Of course this isn't the easiest place to effect a repair, but thankfully this is an A/C car, so with the inner fender well removed, I could sit in the wheel opening and cut the line below the pin hole, put a double flare on it (actually two, cause I forgot to slip the flare nut on the first time), installed a coupling connected to a new 12" section of line.

Button everything up, re-applied the air, checked for leaks, found none and proceeded to walk around the car starting at right rear, did both valves on there, then the left, then fronts, all done!

Hopped in and finally had dynamite pedal pressure! Time to go for a spin!

 

Good news.... Glad it all worked out for you.....

Now, enjoy the ride

 

Glad to hear you solved the problem......one of those that can drive you crazy because it is not visible.