OK.. back to basics. Let's take it form the top.

For no particualr reason - except that the car was apart - I put this kit in my 1968 brake master cylinder.

http://www.zip-corvette.com/ProductD...GR-SR&CTitle=&

I wasn't looking for any problems nor did anything appear unusual when I did the job. Pretty easy in fact - even though I'd never taken one apart before.
Not counting the boot and the clip, the damn kit only has three parts and they looked just like the ones that came out. I don't see how even a MC virgin like me could screw that up - right?

After farting around with my brakes all day and trying a "gravity bleed" I noted the following: I went through 4 reserviors full of fluid in the rear and had good gravity flow out all 4 bleeder hoses - and nothing moved in the front.

I finally took the front brake line off the MC that goes to the "proportioning switch" and I noted that nothing would run out the line hole on the MC via gravity. Now, if I pumped the pedal I could indeed make it go "squirt-squirt" just fine. But no gravity flow at all. - (Independant of any bleeding of the MC to remove air.. the thing STILL outta gravity flow (or not) - right?

There are three holes in the floor of the front reservoir. A very small one in the front, and 2 more larger ones about 1/2" rearward of the front one. It's the front one that "free-flows" into the chamber where the line connets and since it's about the size of a lawnmower main-jet it's easy to see why there's no gravity flow.

So..... do we have a 1968 MC expert in the house that can tell exactly me how this thing works - and if I outta be able to gravity bleed this particular MC in the front?

I'm open to all guesses.

Yes, I'm gonna bench bleed it while it's off again - but that doesn't speak to gravity flow at all.

-W

 

It wont gravity bleed unless there is no air inside. Make sure that when You bench bleed that the tubes go back into master cylinder deep enough that it doesnt suck any air backwards when cylinder retracts. Sometimes if when it pumps air into resevoir the bubbles can be sucked back in when You release piston. When You go back to bleed system on car, open all six bleeders at once. Then close each one as they start to flow solid flow. remember that the whole line will have to be bled out completely as there will be air at MC. Then do Your individual bleeder with someone pumping brakes for You. Start at furthest point and all should be good.

 

Clams, I've got an original '76 MC in my garage. I'm going to go look at it now. The seal design you linked said 67-76, so I doubt that's your problem. It's possible there was a casting change to MC however.

 

Alright, looked at my 76's original MC

2 holes in the front reservoir. Front hole is much smaller (looks like I could just stick an average paperclip in it), and that is indeed the one that I get air through when I blow on the hardline port with a relaxed piston.

 

Thak you...

OK... so is it possible that that particular MC has a hole too small to easily gravity bleed in the front with DOT-5?

My front hole looks like it would only take a SMALL average paperclip.

-W

 

Are the rear reservoir holes the same dimensions?

 

I'll have to siphon it out tomorrow to look. I tend to think not though.

If the rear hole is bigger.. I don't think I'm going to take the MC apart for nothing. I''ll just come up with a new way to move the fluid - like Vacuum.

-W

 

Yes the front hole was smaller and was the whole problem. It was too small to gravity bleed in under a week or so.

It's all done and explained in the other thread.

I also took the time to school them on the '68 to '72 factory Master Cylinder and how to properly bleed it.

Thanks so much for the help!

-W