Is there supposed to be something in the hole on the bottom right? It is the front chamber of the MC. It has threads in it and one book shows a "stop screw". I have the chassis and overhaul manuals and none of them show this in detail. This is the 346 MC for 68 Z.

Yes, a stop screw holds the front piston in it's proper location. It's a screw with an unthreaded protrusion that stops the piston. You could make one by grinding off the end threads of a bolt that fits that thread. It should stick down into the bore by enough to hold the piston but not interfere with the spring. I"d guess 1/8" or so.
David

Thanks David. I need a blow up of the MC. I had it rebuilt and this was missing so now I don't trust any of it. It could be alright, I've forgotten to put in a screw or two, but I need to check. I had the booster restored, new brake lines, and have spent a lot of hours cleaning and painting old parts. I don't want to bleed all this and have to take it apart. Anyone know a source that shows breakdown of this MC? The Chassis overhaul doesn't show this one. It shows the non-power one if that's the same.

I never see the screw in rebuilt masters, but they seem to work fine. I put it back in when I rebuild if it had one before.
It makes it a little easier to get the primary piston and snap ring back in.

I recently got a 346 cast from a 77 vette which has no stop screw. I believe it to be an unmolested cylinder, OE, but I could be wrong.

edit: and the non-power master picture will work...

I e-mailed the rebuilder/seller and he said most cores don't have one and they don't use them anyway. Shouldn't be a problem. I was about to put everything back together when I noticed the open hole with a thread and looked in the repair manual and saw a screw was suppossed to be there. I didn't want to fire it up and have to take it apart again.

I'd have to look at mine but it seems it would allow fluid from the rear chamber to leak back into the reservoir.

The primary piston spring pushes the secondary off the stop screw as you install it.

I'm not sure what you mean, David. Fluid is supposed to flow freely through the breather ports (filler ports).
http://img341.imageshack.us/img341/2435/portxb4.jpg

So scott am I ok without it?

Always worked for me. The set screw makes it easier to install the primary piston... that's the only use I see for it.

OK, thank-you