Trying to figure out what to do next, I'm tired of getting my 67-year-old body contorted under the dash.

From the search on every article here, the conversion between manual brakes to power and or power to manual has been covered at length. The one thing I'm still fuzzy on is the pedal box. From pretty much everything I've read in the Google search, you simply need to use the correct hole for the clevis pin and use the correct brake light actuator arm that bolts to the pedal. Some say that I need to actually put a power brake pedal box to replace my manual pedal box AND use the correct holes. Most others say you simply use the right hole and get the right actuator arm.

So which is it? And since I need a different actuator arm, where can I find that, not seeing it in the normal parts houses? To me, I don't see much difference in the boxes other than the stud and different hole locations for the booster. If you simply drill out the holes and drill new ones for the upper booster studs it all looks identical. And it seems that I'm also wondering if my '71 even used two different pedal boxes. These cars were made for 15 years, and I cannot find anything in the AIM that shows a different P/N for the pedal boxes (ill need to go through it again)

In this picture, mine looks like the one on the left. Even the brake light actuator is in the same position, so I had manual brakes at one time, as the actuator is in the manual brake position. But is this how they did it in '71, or were these separate pedal boxes from later cars?

Again, I'm coming into this blind. I didn't realize the car was converted improperly. And I really have no idea how the PO drove it this way for 7+ years.

Also where to find the PB actuator arm?

 

I am doing this MB to PB conversion in my frame off, so I am familiar with the changes.

There are some minor differences in the bracket holes, on the pedal box, where the MC attaches.
A MB car only uses 2 bolts ( studs actually) to hold the MC.


Nut at A holds bracket to firewall on MB.
Nut needs removed to fit two lower PB booster bolts.
Loose square shoulder studs at B held MB MC. Not used on PB. Needs to be removed.
PB needs a new set of holes at C, for two upper bolts on PB booster. These can be drilled.
Upper photo is labeled incorrectly. The MB MC rod uses the upper hole for more pedal leverage. The lower one is empty, as shown. This setup gives you more foot pedal travel, but lowers the pressure, for the MB system to work. It also uses a 1.0" bore MC.
The PB setup MC rod uses the lower hole, for less leverage, and foot pedal travel is reduced. And the PB system uses a larger 1.25" MC bore, which without the booster, that combo would yield an impossibly hard pedal, without a booster. This can be relocated by you without any new parts.

The above mods could be made to a MB pedal box, to turn it into a PB one. Or you could buy a PB pedal box. Both options will work.

None of the above really require any new parts.

I have seen several odd non-correct combos of the above.
If the system was converted improperly without moving the clevis rod, either the pedal is rock-hard (MB), or the (PB) pedal is way overly sensitive.

This stop switch bracket is the only real part you need to buy. Corvette Central has it.



The hole needs to be cut differently thru the firewall. If the PO did it, make sure he sealed it up properly against heat/water intrusion into the cabin.

Have fun crawling under there.
Seat removal and a long rubber pad or blanket make the upside down position much more comfortable!

 

Whoever did the install, did the FW hole very nice, I had to open up the booster stud holes a little trying to get the new one in was super tight. But the clevis is in wrong hole at least according to the pics. With the new booster I can’t get it to tighten with clevis where I took old booster out( that was damaged and cracked by whoever installed it). So all I need to do is remove the manual actuator arm and put clevis there. Then get new actuator arm for power booster correct? No swapping pedal boxes correct?

firewall

 

Wow that booster just does not look safe.
Glad you found it.

The clevis on the booster rod basically just determines the high pedal or start-up position, while at rest.

 

Well for sure it was leaking vacuum😳 so no power brakes. If I hook up the clevis pin first I cannot tighten up nuts on booster unless I destroy that broken section I showed to get more movement. That’s why I’m 99.9% sure that the clevis needs to go where the brake light actuator is currently. The only way the booster I took out could have been installed was to break it like the picture shows. I’ll order the PB actuator and take it from there.

 

Suggest, if you've not done so already, Remove driver seat from car Before you begin work behind/below dash. That'll pay dividends.

 

Hmmm- Ive seen those pics before! That look like my old garage floor!!!

Anyway- the box on the left is off a 71- the one on the right is off a 75 (restored by David Howard RIP)

I ended up going with a manual brake- smaller master cylinder (7/8") and larger 14" 6 piston on the front- 12" 4 piston on the back- and I love the feel.
Not at all mushy like the OEM-

 

Since the clutch style brake pedal is identical with two holes to put the clevis and brake light striker plates, and two different brake light striker plates, it was designed to be able to NOT change out the pedal box if converting. To me it seems that if you have either of the two different pedal boxes installed, you can go back and forth between manual and power booster brakes using either pedal box by simply using the correct hole and correct striker plate. That's all I'm really looking for a answer to, so I can install the booster in the correct hole and I've ordered the correct striker plate.

 

If you plan to use the manual brake pedal box you currently have, you’ll need to modify it as Leigh stated above and not just move the location of where the clevis mounts on the pedal.
You’ll need to drill two new upper holes at the proper locations so the PB booster studs will align with all four holes.
You’ll also need to remove the welded on nuts for the two lower PB booster studs.

The other option would be to replace the manual pedal box with a power brake pedal box.

I believe the two pedal box assemblies were sold as complete assemblies and the box themselves were not listed as individual parts in the AIM.

Also like Leigh stated, check the bore size of your master cylinder to make sure the master has the correct bore size.

You’ll also need to measure the piston rod length and set it for proper booster pressure.

To order the correct master cylinder you’ll need to double check the pedal ratio.

Here is a picture of the plate I made to cover the different holes in the firewall to keep heat and moisture out of the interior.

 

There're different ones you can use. Distortion boxes, reverb, wah-wah pedals, stuff like that.

Oh, we were talking about cars, sorry.