Vet65te

Got a few questions for anyone who's recently reassembled their 65/66 PB Master Cylinder, not had it done by others. My two midyears, 65 & 66 Coupes, started out as non-power brake cars. I added PB to the 66 a few years ago with the technical assistance of Vark_WSO in Wisconsin and that setup has worked just fine ever since.
Now, I'm attempting to do the same mod to the Black 65 Coupe. I already had most of the hardware stashed for this event except for a particular 3/16-inverted flare elbow I needed but found a vendor in Las Vegas that has them so that's covered. By the way, I'm not using the factory brake line routing.
As for the Master Cylinder for the 65, I have the one that came out of my Dad's 65 Air Coupe that was sent to SSBC in New York way back when we were refreshing that car (~1992) and intend on using it on my 65. Back then we sent it out for sleeving but for some odd reason couldn't get the system bled down afterwards. We wound up using a generic '67-on MC and that worked well and stayed in place on that car so that's why I have the original 65 MC still around. We did send it back to SSBC who sent us a polaroid (remember those) of it on their test stand exerting something like 1100 pounds of pressure. Got it back and just stashed it for another car. It's been on the shelf till now. Oh, since the old brass line seats were never replaced in that previous rebuild and looked to be a bit scarred up, I ordered a pair of new ones from CSSB, the folks who sell the repro 65/66 PB MC.
So, after taking my MC to clean out any debris and coagulated old brake fluid, I was reassembled it but found that after I had it back together again, the piston travel sure seemed short, much shorter than it should be. I did compress the front piston before installing the piston stop bolt but when looking down through the chamber, it seemed like the piston was moving rearward more than it should have been.
Pulled that piston stop bolt back out because it didn't have any effect on that front piston. Now, back when we sent this MC to SSBC, they said we could keep the caps and internals since they were going to install new. So, check out the pic of the 'new' piston stop bolt on the right sitting alongside the 'old' bolt we kept from the original innards for this MC.

Don't know if that had anything to do with the lack of pressure when trying to bleed the system way back when but it might have.
But wait, there's more...and this is why I need to talk to someone who's had recent experience with reassembling these master cylinders.
I understand that other than the differnence in the casting between 65/66 and later versions, the brake piston activation travel should be the approximate same distance.
When that front piston is depressed and the old bolt is in place, and you let the piston travel back up against that bolt, there seems to be very little overall travel, like maybe 1/8-inch tops. Don't know if that's right or not. Then, when the rear/larger piston is in place and captivated by the circlip and you try to push on the full piston assembly, the overall travel is maybe a half inch, 5/8-inch tops. Travel dimensions don't exist in the service manuals and I'm hoping someone can tell me if that shortness of travel is to be expected or something is amiss.
Thanks,
Mike T.

mashinter

I get 1 inch on my '65 M/C

Vet65te

Thanks, I think that's about what I should be seeing on this 65 MC too.
Right after I posted, I pulled out a dual MC that's going to be added to my 56 Nomad (yes, too many projects, too little spare time) and when I depressed the piston assemblies on that one, I saw it move at least a full inch. Okay, one question answered. Now I need to figure out what is interfering with the travel.
Mike T.

Vet65te

Just pulled the piston assemblies out of my 65 Master Cylinder and found that while the 'old' piston stop bolt never stopped anything, the new one, even with that slightly longer reach, is still too short to effectively work as a piston stop. After removing the pistons I installed that new/longer stop bolt and could just see it barely protrude into the bore. Take a look at the front piston and you can just see the horizontal gouge that new bolt left just behind the front seal.

Mike T.

mashinter

The bolt is supposed to go behind the front piston. You are supposed to compress the front piston and then install the bolt. Is that why you're only getting a short travel?

Vet65te

The front piston was compressed forward and the 'longer' stop bolt installed as per the 65 Service Manual Supplement. Then when I took the pressure off that front piston, I could see (through the tiny hole in the floor of the front reservoir) that the piston seemed to back up more than I would have expected. That's why I removed it from the master cylinder and found that the short reach of this 'longer' bolt was not enough to hold the piston as designed but just long enough to score the outer edge of the piston as it moved rearward.

So, now it appears I need to find a longer stop bolt. The one added by SSBC when the MC was sleeved was definitely not long enough but this one I just tried to use was the one that was in place when we took the MC apart back in the early 90's and I would have thought it was the original bolt but that's only a guess because we towed my Dad's 65 Coupe home and never did get the chance to try the braking system before we tore the car apart.

In any case, I now have to see if I can dig up a longer stop bolt if I want to put this master back into original configuration but it also makes me wonder if the folks at SSBC used the shorter bolt just to plug the hole since it obviously had no chance of really protruding enough to stop that front piston.

I just opened up that other PB MC I have stashed for my Nomad Project (67-later style MC) and see that it doesn't use a stop bolt.

Mike T.

mashinter

I get 0.465 in. for the shank length of the bolt in my '65 M/C

Vet65te

I measured the shaft length of the longer bolt which is the one that was in this 65 PB MC when we got the car in the early 90's and it was .465. I also measured the shorter one installed by SSBC when they sleeved this MC and that bolt is .423.

I've dug up some additional info on this setup from the people at CSSB in Miami, the ones who sell the repro 65/66 Power Brake Master Cylinders .
They carried the 3/16 brass seats I needed and I asked their tech about the stop bolt issue and he said it's only used when assembling the pistons assemblies. I believe that's correct because when SSBC sleeved this MC they installed the shorter bolt which didn't extend down into the piston bore at all. It was doing nothing more than plugging a hole in the floor of the reservoir.

Mike T.

mashinter

I am guessing that the bolt has a function in the operation of the M/C. Maybe the front piston will pull back too far and move the seal behind the fill port? The seal moving across that large hole might trash the seal in short order. Dunno.

Vet65te

That was my guess too, that the stop bolt was in there from the factory for a reason and it should still be part of the overall design 'but' so far I've run across two businesses that deal primarily in Corvettes Brake parts and systems (CSSB & SSBC) and both do not use that stop bolt as a stop bolt. Here's what Luke at CSSB wrote on the subject of that stop bolt.

The stop bolt is not needed, it was used to help assemble the master cylinder. It does help in some cases to keep the piston from traveling back too far. The pistons seal should be parked just behind(towards firewall) the small port. What this does is push any air out of the tiny hole then produces pressure. If the hole were bigger, the seal would be extruded through the hole as it passes it. The hole is called an emergency bleed port, this is the only way air can leave the bore.

Here's a pic of the reservoirs on my 65 Master Cylinder.


Granted, he does say that in 'some cases' it does keep the piston from traveling too far rearward but for now, I'm going to reassemble it with the shorty stop bolt that merely plugs the threaded hole. Of course, once I have the power brakes installed and am trying to bleed it and find it won't bleed down, you'll all be able to hear me cussing big time.

Mike T.

mashinter

That small port is called a compensating port, and its primary purpose is to allow fluid to expand back into the reservoir. Without that port, when the fluid expands due to heat, the pressure will apply the brakes. That will heat the fluid even more, and your brakes will proceed to apply harder until your car grinds to a halt without you touching the brake pedal.

I learned that one the hard way. My compensating port was plugged.

JohnZ

Do you have the '65 Shop Manual? It has a detailed cross-section of the master cylinder on page 5-6 that clearly shows the stop bolt as keeping the floating piston from traveling too far rearward, while still allowing it to travel full forward under fluid pressure.

Vet65te

And I clearly stated, post #6, that I had the 65 Service Manual Supplement and that's the sequence followed when I reassembled the 65 Power Brake Master Cylinder using both the original stop bolt and the bolt installed by Stainless Steel Brake Corp in New York when they resleeved this master cylinder. Apparently both companies feel that stop bolt is unecessary.

mashinter

I was able to pull the bolt and re-install it without compressing the front piston, so it seems it does nothing in an assembled M/C.

It doesn't make sense that it would be used as an assembly aid.

Maybe that stop bolt is there in case you break the rear spring. It would keep the front piston from retracting too far due to the unbalanced pressure.