While looking for other information last week I came across a post here that mentioned different depths in the master cylinder where the push rod seats. I was getting ready to install mine tonight and I hate to say it, but it looks like I've got incompatibility issues. I bought a '68 Corvette master (it's going to be an all-disc system) from the parts store, and got a new booster online. The rod from the booster hardly sticks out - maybe half an inch. The depth of the hole in the master cylinder is 1 5/8 of an inch.

Is this a problem? From what I've read elsewhere it seems like I should have either a master with a shallow push rod hole, or a booster with a longer push rod.

If I do have a problem, what are my options for getting it working? I imagine I could go buy another MC (bought this one months and months ago, and I'm pretty certain they won't take it back), but I'd at least need a part number or application to give to the counter guy. Is there a way to lengthen the push rod, or obtain a longer one? Can I fill the hole in the MC with a fitted piece of steel?

How did you measure this? Remember that the m/c seats into the booster, so you have to find a good point of reference when you measure it. I measured my short pushrod before I put it in, and it was 2.605". I have a long pushrod somewhere, I can probably find it and measure it. Just send me a PM if you need the length. I think the clearance between the pushrod and the piston should be something like 1/16" or so (if someone who knows reads this, please correct me). Anyway, it's equally important that it's not too long (=won't let the piston all the way back, and the brakes will drag). You can check it by using modeling clay, just like you check valve clearance in an engine. It sounds like you need the longer pushrod, but your best bet would probably be to use the modeling clay method to determine how long it really needs to be, then go to your local GM dealership and ask if they can get it. Places like MP Brakes should carry them too. I have seen references to adjustable pushrods, but have never used one and don't know where to get them.

Thanks, Olle. Let me see if I can offer some clarification.

The end of the rod (at least that's what I'm assuming I'm seeing) only sticks out of the front of the vacuum booster *maybe* half an inch. Possibly less.
I put a 1/4" ratchet extension into the hole on the back side of the master, marked it, pulled it out, and measured it. From the edge of the hole to where the extension bottomed out it measured 1 5/8 of an inch.
Unless that pushrod extends a good distance within the first few degrees of pedal travel, it seems like the rod won't contact anything until it's nearly at full travel. It's gotta have nearly a full inch of clearance as it is.
Is there a longer rod that I can install in the booster? Thanks for the tip on MP Brakes.

I had the same problem, there are at least 2 different length pushrods for these cars. The booster I used had a short rod, and another junk booster I have had the long rod, so I swapped them (easy to do, they just pull out)

I also have a '68 vette MC

Derek,
Thanks for the reply. I just spoke with the tech at the company from whom I bought the booster. He confirmed what you stated. He said the model of booster that I bought (it's a 7" dual-diaphragm booster) only comes with the short rod, and it is not able to accept the long rod. Oh, by the way, I learned from the MP Brakes site that this rod is called a "booster pin," as opposed to the actuator rod that attaches to the pedal itself.
At any rate, he said I could either buy another master, or I could install a proper size plug in the existing master so that the depth is correct.

I'll be pulling mine apart again and can compare lengths so you know how long of a plug you'll need, but I didn't plan on getting to that part of the project for about 3-4 more months :clonk:

Thanks, Derek. Hey, it may work out. After all, I didn't plan on MY rear end rebuild turning into a near-total replacement of the brake system spanning a year! :clonk::clonk:

I heard thru the grapevine that Baer sells a pin extention. I know some people have just made them out of bolts, but you have to get the length and shape right.

For a master with a shallow hole for the short rod...
1977-82 Corvette master... looks just like the one you have for the 68.
Also 1970-80 Camaro has a rectangular disc/drum master with shallow hole. Both are 1 1/8" bore.
There are others, some even with correct 67-8 casting #s if you risk a rebuilt unit.

chevry,
Awesome! :hurray:
I called Baer a couple minutes ago. They do have the part, and here are the details should anyone else be searching these posts in the future:

Baer Brakes
Master Cylinder Pushrod Extension
Part #6150001
Price as of 11/30/05 is $8.35