68 C40 pizza truck Brake issues

[Unofornaio]

HERE ARE THE SYMPTOMS:
1: PRESSING the pedal produces a hissing sound (clearly coming from booster)
2: The vac gauge dropes to zero in concert with the hissing.
3: When pumping the brakes the pedal gets hard and STAYS hard.
4: The initial push of the pedal goes to the floor with the same resistance characteristic of air in the lines, some resistance but pedal is loose and goes to the floor if I push it there.
5: There are NO visable brake fluid leaks.
6: There has been NO LOSS of brake fluid from the resivor.
7: All lines are clear and flowing.

I was going to remove the master bolts and peak at the cylinder just to see if there is fluid but again I have had NO loss of fluid. Meaning the resivor level has not dropped.
One more thing the brake set up and components are all the stock components nothing has been changed or modified.
Single pot master firewall mounted booster and air resivor under driver side behind running board/step.

Thank you

[Unofornaio]

Have I been secretly black balled or what?

Just about every Chevy truck from 67-72 had this same brake setup as me.

[Bob B.]

Hissing booster is not normal, may have a blown diaphragm. This can cause the vacuum gauge to drop to zero when applying the brakes as well. The tank behind the running board is a vacuum reservoir. As for the sinking brake pedal, could be air in the brake lines. These systems usually need to be bleed with a pressure bleeder because it is hard to get all the air out of dual rear wheel cylinders. All four wheels and drums should be pulled and the brakes completely inspected and adjusted, sometimes excessively worn drums and shoes can feel like air in the brakes lines. Since this is a medium duty truck and it has a single circuit master cylinder, if you are not completely familiar with how the brakes work on it you should have a competent medium/heavy truck mechanic check it out. Good luck, it looks like a nice old truck.

[Unofornaio]

Thanks Bob after no replies I cut all the info off my original post to get to the point quicker.
What I've done is go through the entire brake system. Replaced all the wheel cylinders inspected and measured the drums and shoes all we're with in .038 of new. Flushed lines put everything back together, adjusted and bled. Another member stated that I skipped a step which was pressing the pedal to center the shoes and I should still hear a slight intermittent sound after that and not back off like I did. While I I believe this is the correct procedure despite this I am now convinced it is the booster.
But once again finding the part is going to be a journey. I'm fairly certain I found them at Napa but will not know for sure which stroke I have on it until I pull it.
Thank you for taking the time to reply I do appreciate it.

[Nobighurry]

I replaced the booster on my 1970 c60 with a rebuilt one from napa. Found the part number stick on the original and just had then cross reference it it was like $180 or something like that. Remarkably, NAPA is a great resource for locating old parts for these trucks. But you have to find a store that knows how to use the old paper books not just the computer.

[Unofornaio]

Originally Posted by Nobighurry

I replaced the booster on my 1970 c60 with a rebuilt one from napa. Found the part number stick on the original and just had then cross reference it it was like $180 or something like that. Remarkably, NAPA is a great resource for locating old parts for these trucks. But you have to find a store that knows how to use the old paper books not just the computer.

Thanks. I found there are apparently 2 different pedal rod lengths for the old Midlands on this year. But I spoke to Power Brake Sales in Sacramento ( one of only a hand full of rebuildres left in the County) and they told me they ship their units WITHOUT the rods and I would simply re use mine.
So I found a few at Napa but they list different lengths 6 3/4" and 8" but a big difference in cost between them. 359 for the 6 and 169 for the 8
When I spoke to Power Brake I mentioned I did not know which rod I needed they told me it's the same unit and that is not an Issue as I just re use mine.
Now that seems to make sense and my brother swears they are one of the few places that actually know what there talking about.
Is the distance from the interior firewall the only factor in pedal rod length?
It sounds reasonable but if the C40s all have the same pedal and distance from the firewall why the 2 choices?

[txagincali]

I am not sure on the rod length question you have, but I saw that you were in Bakersfield. I am pulling the cab off of my C50 and will have the chassis, fire bed utility truck bed, both axles (rear is 2 speed) with wheels and brake components available. If you are willing to have it towed, you can have the rest of the truck if you want it.

[Unofornaio]

I'm going to send you a DM. I am interested thank you.