This part is amazingly difficult to find, but often should be replaced. I've only found them at Corvette resto vendors, and priced accordingly, unfortunately.

1964-1967 Corvette PB Shell Front Seal Original GM 5464287 or 5452549

Corvette Power Brake Booster Front Seal.: 1964-1967 - FREE Shipping

 

After a lot of searching I found the same seal that joe found, and it seems to cost about $33 + shipping from every place that carries it.

I decided to bite the bullet and buy a new booster to go along with the master cylinder. The booster was about $95 at NAPA and they had it in stock, so its already installed. I will be back on the road as soon as I install the new master cylinder and bleed the brakes. It got dark too early to keep working outside today.

Thanks again.

Bruce

 

Updated info

I got the new booster and master cylinder installed, and then read through the brake bleeding process for my truck. Some of this info might be useful.

- There is a release button on the proportioning valve and it has to be pushed in while bleeding the brakes (I think that deactivates the valve and you get equal pressure to both sides). My manual showed a GM tool/bracket to hold the button, but I was able to use a small clamp from HD (the type you use to hold two pieces of wood together) to hold the button down.
- The GM process is to start bleeding at the wheel closest to the master cylinder and work toward the one farthest away, since there is no sense trying to make an air bubble travel further than it needs to. This is the opposite from what I have always heard about starting at the farthest away wheel, but it makes sense.
- Bleeding went well the first time around, but my wife told me that the pedal was still dropping after I shut the bleeder. I thought she was just not used to the "feel" of the brake pedal, but when I got done I found the same problem, so I decided to bleed them again.
- I started out go through the bleed process again, but then noticed a drop of fluid around one fitting on the master cylinder. When I sat in the truck pushing the pedal down hard, and looked out under the hood I found that under high pedal pressure it was seeping around one fitting. I took it apart and cleaned the flare surfaces, and finally got it to hold.

Lesson learned: On the first round of bleeding we had one person in the cab and one under the truck, and I was checking at the calipers and drums. but nobody was looking under the hood for leaks at the master cylinder. It surprised me how much that very small seepage created an extremely low pedal.

Bruce

 

I'm usually the first person to say "read the factory service manual", but in this case I've been bleeding GM brakes for over 40 years. I have never held the button on the combo valve while doing it and never had a problem. Second, I have GM factory service manuals from the early 1950s to 1999. They are about split on starting at the closest or the furthest. I have yet to hear why this matters on a dual circuit brake system. When you have a split system, bleeding the front brakes has zero effect on any air in the back brakes, and vice versa.

The one thing that absolutely matters is to be sure you don't push the differential pressure switch piston all the way to one side in the dist block or combo valve. This is the single biggest cause of problems people have when bleeding brakes (well, maybe second biggest behind failure to bench bleed the MC). There is an inexpensive tool that screws into the combo valve/dist block and temporarily locks the piston in place.

 

I think we are on the same track, but maybe my terminology is off.

My truck has a combination metering valve, warning switch, and proportioning valve. I have read the description several times, and it still looks like the metering valve (allow more pressure to rear brakes) has the opposite effect as the proportioning valve (prevent premature rear wheel lockup), but somehow those functions work together.

The GM repair book says "The combination valve, . . . must be held in the open position while bleeding. This can be accomplished by installing tool J-23709 . . . and pushing in on the pin on the end of the valve." The tool looks like an L-shaped piece of flat metal that is attached to a mounting bolt for the valve. It wraps around the end of the valve and pushes the piston down. It would have been fairly easy to make one, but a C-clamp also worked to hold the piston down.

Thanks again.

Bruce