power brake conversion, booster push rod seems too long

[vetnat]

1961 GMC half ton, converting from stock brakes to dual circuit power brakes. Using CaptainFab's bracket. Got a booster from the junkyard, from a '97 Chev 1500, using a new master cylinder, for a 1978 K10. Using the proportioning valve off the junk yard 78 K10. Also converted the brake pedal linkage using the mod suggested by chevyrestoguy in his build thread (#324 in http://67-72chevytrucks.com/vboard/s...+pedal&page=13) . Bench bled the master, made my own new brake lines from master cylinder to proportioning valve, and from proportioning valve to rear brakes. Am going to convert front to disc brakes, so for now have the ports on proportioning valve that go to front brakes capped off, just working on getting functional rear brakes so I can drive truck into garage for now. Also have the proportioning valve bleeder tool installed. Was starting to bleed the brakes, when I discovered my mistake.

Now here is where things get stupid, my only excuse is I am a newb when it comes to power brake boosters. Turns out, the booster I got from the junkyard was missing the push rod that goes to the master cylinder (as well as the front seal and plastic cylinder the push rod fits into). Basically, just had a big hole in the front, instead of the black plastic seal with push rod protruding. Like I said, never having seen a booster without the master cylinder attached, I just didn't know what the front of the booster was supposed to look like.

So, back to the junk yard I go, get the front seal, push rod, and plastic cylinder the push rod sits in (not sure what this is called). Pulled these parts from the booster of a random 88 to 98 Chev 1500. All 3 parts can just be removed with your fingers, so I'm actually not certain that these parts just didn't fall out and get lost while transporting and storing the booster before I installed it. Installed the missing parts into the booster which is already mounted in my truck.

When I was re-installing the master cylinder, I noticed the push rod was making contact with the master cylinder piston before bolting it down. When I did bolt the master cylinder up, it squirted some brake fluid out the finger- tight front port brake line. The push rod is non-adjustable.

So, should the brake booster push rod be interchangeable between different boosters of the same type? Or is each push rod "calibrated" to the individual booster? Should I unbolt my linkage at the brake pedal to make sure the linkage isn't applying pressure without any pressure being applied to the pedal? Any other reasons the push rod would be too long that I haven't though of? I realize that getting a new booster (comes with a push rod) might be the way to go, but I would hate to shell out for a new booster, and still have the same problem. Thanks for considering my stupid newb problem.

[mongocanfly]

If your local parts store has a booster like yours just compare the 2 rods...if yours is to long I'd just trim it to fit/work....part of your issue may be all the diff yr model parts your using...

[vetnat]

Thanks Greg, that’s a good idea. The combo of 97 booster and 78 master cylinder is supposed to be compatible according to documentation that comes with power brake conversion bracket from captainfab, so that’s why I’m wondering if there is something wrong with my push rod, or my linkage.

[mongocanfly]

captain may know....

[chevy_mike]

One thing to note, with master cylinders, there are 2 different depths where the push rod goes into the MC. Commonly these are called "shallow" or "deep" Depending on what the booster is designed to use will determine which one you need. They are not interchangeable.

My guess is you have a shallow type MC with a booster that uses a deep style. This is why the rod touches before tightening. Post up a pic of the MC piston and the push rod installed in the booster. That might help confirm.

[The Rocknrod]

chevy mike is right.
This thread 'may' help explain it - http://67-72chevytrucks.com/vboard/s...d.php?t=498293
Pushrod adjustment article - http://www.classicperform.com/TechBo...ot.htm#pushrod
And ensure you pedal is drilled properly - http://www.classicperform.com/TechBo...hoot.htm#pedal

[Captainfab]

That booster and master cylinder should both be compatible as they both use the short intermediate pushrod. However not knowing just what pushrod you installed since the original was missing, may be part of the problem. I would suggest first disconnecting the pushrod from the pedal, to be sure that it is not pushing on the booster. Technically there should be .020" clearance between the intermediate pushrod and the dimple in the back of the master cylinder piston. You might also check to be sure that the intermediate pushrod is seated correctly in the booster.

[vetnat]

Thanks for all the info guys. Yes, I sure picked a winner of a booster to pull at the junk yard. There are plenty of 88-99 trucks at the yard, but it sure is not very much fun removing the booster, getting at the nuts up under the dash

It is supposed to be only -2C here today, and I scheduled the day off work, so will see what I find out. Thanks again.