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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!
Thanks Leigh!
I’m already thinking about how to cover the old unused holes.
I’m thinking some type of cover plate, that will fit right up under the master cylinder gasket, but we’ll see.
I could do some fiberglass work, but I just finished painting the firewall and don’t want to start painting on it again.
I would greatly appreciate you slipping your old gasket into an envelope and getting it into the mail.
Do you still have my mailing address?
You’re a lifesaver,
Thanks guys,
Greg.
Hey Leigh,
We seem to be typing over each other.
Ok I see what needs to be done here.
I won’t need your old gasket, but thanks for the offer.
If I was to make a cover plate that fits tight up to the master cylinder and don’t have any fiberglass that covers the old section of the main hole, will that space cause any problems with the function of the master cylinder.
Does the gasket only act as a cushion between the master cylinder and the firewall or does it actually seal something?
I realize that the diameter of the hole (and it’s location) for power brakes is dictated by the circumference of the lower opening in the mounting plate, but could you confirm the actual diameter of the hole by measuring it (before you cut the new hole for manual brakes)? I’m going the opposite route (manual to power) and just want to be sure before I start cutting.
Greg
I would think it is both a water and hot air seal, to keep that stuff out of the inside of the car.
The square bottom of the booster bracket will hide it anyway
Just seal it up somehow.
I see a gasket on this too.
OOPs you are going the other way.
The M/C base for MB is much smaller than the PS booster base.
You are going to have to fill those holes and seal them somehow.
I am sure you don't want those holes to show.
The 2 upper left over screw holes will likely be the most obvious to see.
Maybe a neat black painted and silicon'd sheet-metal patch plate?
Home Depot has that stuff.
Let me know if you change your mind on the gasket.
Last edited by leigh1322; Nov 22, 2025 at 03:42 PM.
Greg
I would think it is both a water and hot air seal, to keep that stuff out of the inside of the car.
The square bottom of the booster bracket will hide it anyway
Just seal it up somehow.
I see a gasket on this too.
OOPs you are going the other way.
The M/C base for MB is much smaller than the PS booster base.
You are going to have to fill those holes and seal them somehow.
I am sure you don't want those holes to show.
The 2 upper left over screw holes will likely be the most obvious to see.
Maybe a neat black painted and silicon'd sheet-metal patch plate?
Home Depot has that stuff.
Let me know if you change your mind on the gasket.
Leigh,
I’m thinking of making an aluminum patch plate that would cover all the holes, then drill new mounting holes and a new main hole through the cover plate for the correct MB location.
My only concern is:
How thin would the cover plate material need to be before it would affect the length of the push rod for the master cylinder?
I could make a cover plate that would cover the lower portion of the main hole and fit snug up to the base of the master cylinder and use a couple plastic plugs for the upper mounting holes.
This option would have no affect on the push rod.
Again I’m probably over thinking this.
Also, can you drop the gasket in the mail?
New they are under $5.00 but everyone wants $12.00 to ship it and the local parts stores can’t figure out what I need.
Edit: I haven’t ordered the new master cylinder yet so I have nothing to compare it too
Last edited by OldCarBum; Nov 22, 2025 at 05:45 PM.
I realize that the diameter of the hole (and it’s location) for power brakes is dictated by the circumference of the lower opening in the mounting plate, but could you confirm the actual diameter of the hole by measuring it (before you cut the new hole for manual brakes)? I’m going the opposite route (manual to power) and just want to be sure before I start cutting.
And I need to correct what I said about the pushrod clearance.
That spec & tool is for a PB booster.
My MB car has a solid pushrod.
Any extra clearance would just raise or lower the pedal a smidgen.
IIRC the pedal has 7" of travel.
Even 1/16" should not be a problem.
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
I did a right up on the conversion many years ago for a 68. Its very easy and you just use the existing metal in the pics as a template. Knockiing the old blind nuts out was pretty easy as well.
here it is...starts at post 7 https://www.corvetteforum.com/forums...onversion.html
Today I cut the new hole in the firewall and drilled the two new upper mounting holes using the metal plate on the interior side as a template.
I used a fine toothed hacksaw blade to make the rough cut then I used a half round file utilizing both the flat and half round sides to get the final shape.
Then I took a trip to Ace Hardware and purchased a piece of 16 gauge steel sheet to make a cover plate.
Once I receive Leigh’s old gasket I’ll cut the sheet metal to size, cut and drill the holes for the manual brake master cylinder and bolt the plate to the firewall using the upper and lower pedal assembly mounting hardware.
Today I had the time to measure the cover plate and cut it to a 5” by 5” square.
Then I measured the drilled holes in the firewall, compared them to the pedal assembly bracket and marked the cover plate.
I used the master cylinder gasket I received from Leigh and centered the main hole.
I had to order a new jig saw from Eastwood so I can rough cut the main hole.
When it arrives, I’ll drill the mounting holes, cut out the main hole, paint the cover plate and install the pedal assembly.
Thanks again to Leigh!
Excellent thread. I'm going manual brakes from power...decided to bite the bullet and put the 14" Aero6 setup from Wilwood on the fronts and finally get that direct, manual brake feel all the racers rave about. This has been extremely helpful as the M/C mounting (adapter plate) has been the biggest question mark.
Just to let you know, I have my manual pedal box all mounted to the firewall with the manual master cylinder mounted on the engine bay side.
Everything installed perfect without any issues.
