Did a quick search and didn't find anything on the differences between a C3 with power Brakes and standard brakes


Power brakes firewall


Standard brakes firewall

 

The metal on the inside of the dash is there for both applications. Just need to fill or cut the glass away depending which way you are converting. I went from power to manual. The pedal box is different for each although either box can be made to work either way. I just went ahead and got a manual box and did it the factory way.

 

Yes, it kind of surprised me. I'm going from std. brakes to power. When I was installing the brake petal assembly and the top holes didn't line up . I was like what the hell is going on. I never have seen this discussed in the 30 years I've been playing with c3. I always thought it was a direct swap, which it really is not. Luckily, the front cap is off and it won't be too much of a pain in the azz to glass and paint.

I didn't know the petal boxes were different. But now I do.

 

I will be doing this also in the future. These are the pics I found of the differences in the pedal box that I squirreled away for future reference. The rod is mounted in a different hole and the brake light switch bracket changes. Also notice the two top studs thru the firewall turn into bolts. The MC rod moves much higher up on the pedal for more leverage on the MB. Results in way more foot pedal movement.

Both the rod and the brake light bracket bolt in different positions. MB on left.


Both the rod and the brake light bracket bolt in different positions. MB on left.


The two different brake switch brackets. MB on left.

Are you sure one of those pics is from a factory PB car? I heard or saw somewhere? that the firewall hole had to be enlarged? AIM maybe?

 

Yes, fist picture, where the hole looks centered between the 4 stud holes is a power brake vette. Was all original, number matching .

I have not checked if the hole od are the same.

 

Oh Thanks for that! I see the difference now.

On one pic the hole in the fiberglass is centered between all four stud holes, and one hole is centered between just the top two. (I seem to remember one guy online who cut a slot into his to make it work.)

At any rate that makes since since based on the pedals, the MC actuator rod is moved up or down on the brake pedal assembly between MB & PB. One is mounted higher on the brake pedal arm, one is mounted lower. But interestingly both holes already exist on the brake pedal itself so that part is exactly the same.

So there are two major mechanical differences between the two systems: 1) MC cylinder bore size and 2) MC actuator rod mounting height.

I recall a couple of guys on here having brake problems and discovering that only one of those two things had been changed. Buyer beware!

Here is my factory MB car below. The two upper studs are still in place which need to be removed from the pedal assembly and turned into bolts for the brake booster. And my actuator rod is still in place, and mounted high on the brake pedal. You can even tell how horizontal it is. And so the MC sits higher on the MB cars and is centered between the top 2 bolts. Thanks to you I have a much better idea of what I need to change to complete the PB swap.

Original 72 BB with MB Car

 

I think Leigh is saying this in his posts, but just for clarification, the two existing bolt holes on the firewall that mount the manual brake master cylinder are NOT in the same location as the upper bolt holes for the power brake booster. The manual brake master cylinder holes are roughly 3/4 inch lower centerline to centerline than the upper bolt holes for the power brake booster. The two bolt holes on the bottom are the same for both power and manual brakes.

I have seen poster on this forum who converted from power to manual and just mounted the manual master cylinder to the existing power brake top two holes, and it must work, but the geometry is not as GM designed and the rod is not going straight from brake pedal attach point to where it belongs.

I converted my 77 power brake system to manual brakes, and had to make a cover plate to cover both the upper two bolt holes, as well as the opening. And I had to drill two new holes in the firewall 3/4 inch below the upper power brake booster holes, and those holes had to be drilled though into the pedal mount. I was able to use bolts and nuts to hold the manual master cylinder on, as opposed to the captured studs from the factory. The switch brackets were changed, and the position on the brake pedal swapped as Leigh described.

I think if you are converting from manual to power, all you have to do is use the power booster gasket, position it to line up with the bottom two bolts, and then drill the upper two bolts through the firewall and into the brake mount, open up the big hole, and then swap the bolt attach positions on the pedal, and make the switch bracket changes.

 

Beware that the upper bolt holes on a power brake box and firewall are higher up than the top two holes on a manual firewall and pedal box. Oops, I see Flyboy already covered it well.

 

Well flyboy you are absolutely right about the upper holes and this is such a good thread on this subject I thought I would help document it in pictures.

Change in upper bolt / stud location on bracket


MB upper Holes exactly in line with body seam


PB upper holes 3/4" higher

 

Ok guys,
I know this is an older but not too old of thread and I will start a new one.
All I wanted to do is make sure I can go back and review this older thread in the near future.

Since I’m going from PB to MB in my 73, what I need to do to install the new MB pedal assembly, is to mount the assembly on the firewall using only the two bottom holes, mark and drill two new upper bolt holes and bore a new hole for the rod directly centerline between the two upper holes?
Correct???
Can someone measure the large rod hole so I can make it factory correct?
Please and Thank You,
Greg

 

OCB
I can measure my car and/or make you a template.
Somewhere around here I have a pic of the firewall cut both ways.
If I can just find it!

 

Thanks for responding Leigh!
If the photos you are referring to are the ones in post #9 above I already have them.
If you can measure your car or even better, make me a template that would be great.
The last template you made for the Z rod bracket worked out perfect.

 

This one makes the mods pretty clear.

 

Here is a good picture of both setups.

 

Yes that pic is much better!
So the two lower bolt holes get re-used and do not have to change at all.
The main hole needs to go up or down.
And the top two bolts also need to go up or down.
And these two bolts go thru both the f/g firewall and the metal bracket, so you have to drill new holes in both the f/g and the metal bracket.
Some fiberglass repair would have to be done to make it really neat and cover up the three old now unused holes.
I can tell which way OCB is leaning!
Neatness counts!

 

I started a new thread on this subject but I will post my findings here as well.

Quote!
As part of my 73 body off project, I’m swapping from power brakes to manual brakes.
I was recently informed by another forum member, JohnRR, that I would need to drill two new mounting holes and cut a new hole for the master cylinder rod to pass through the firewall.
Leigh1322 offered to measure the hole locations for me from his manual brake equipped car.

Today as I was looking in the area where the pedal box assembly mounts to the firewall from the interior side, I noticed that the oem metal reinforcement bracket bonded inside the firewall appears to already be designed to be used for both power and manual brake pedal assemblies.

I held the manual brake pedal assembly up to the interior side of the firewall and aligned the bottom holes in the firewall with the bottom welded nuts on the manual pedal assembly and they matched.
I could see that the top square holes in the pedal assembly matched up to the square holes in the metal plate bonded to the firewall.
Just eyeballing the cut out in the metal plate just above the hole in the firewall for the power brake rod, it appears that the hole in the brake pedal aligns with the center of the cut out of the metal plate.

For all you pros:
Do I just need to remove the area of the firewall to match with the metal plate to properly install the new manual pedal box assembly???

Thanks in advance,
Greg




Sorry for the crooked pictures.
It wasn’t easy holding the pedal assembly, the IPad and the light.

 

OCB I just measured my OEM MB car.
That hole is 45mm or 1-3/4" diameter.
And there is a gasket around it . It goes around the 2 bolts and the hole exactly.
If you could find one you could use that for a template.
Or If you need my old gasket, let me know.


That is my firewall back up in post #6.
You can see the gasket.
And the indentations the M/C made on it.
The gasket is almost exactly the shape of this raised boss on the back of the M/C.
It would be easy to make your own. Template or gasket.

 

So if the same bonded in plate was used from the factory for both manual brake and power brake cars, wouldn’t it make sense that if someone was going from manual brakes to power brakes, all they would need to do is cut out the lower section of the main hole and drill the upper two mounting holes using the metal plate as the template?

 

Yes!

Make a paper template from the M/C boss.

Do you have a new M/B pedal bracket?
The two upper holes should be in a slightly different (lower) position than the PB bracket.
That is a template as well.

One of the pics above the guy must have goofed and drilled another set of lower holes. Don't do that. Reuse the same lower hole.

 

I missed this pic you took.
It's perfect.
I did not even know there was another metal piece bonded to the inside of the firewall.
There is your template.
Cut away.
Cut out all that exposed fiberglass around those 3 holes.

Then patch the f/g up until it is as neat as the rest of your car, and the holes look like mine.