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Joined: Mar 2010
Posts: 10,155
5
'Bolter
Originally Posted by Mike B
Aren't you using the original brake pedal arm and doesn't it hit the lower toe board to stop it's travel? What's the difference between now and when it was a single pot MC? The pedal/arm has to return and bottom out against something so you can adjust the rod to allow the MC piston to fully retract.

Mike B smile
The original brake pedal arm does not hit the toe board. It is held at a specified distance from the toe board by turning the threaded clevis thus adjusting the length of the pushrod.
The problem the OP is having is that, unlike the original master cylinder, his Corvette master cylinder does not have a captivated pushrod. There is nothing to prevent the brake lever return spring from pulling the Corvette pushrod out of the master cylinder.

Joined: Mar 2004
Posts: 5,538
M
'Bolter
Originally Posted by 52Carl
The original brake pedal arm does not hit the toe board. It is held at a specified distance from the toe board by turning the threaded clevis thus adjusting the length of the pushrod.
The problem the OP is having is that, unlike the original master cylinder, his Corvette master cylinder does not have a captivated pushrod. There is nothing to prevent the brake lever return spring from pulling the Corvette pushrod out of the master cylinder.

So if I understand right, the AD trucks have a washer/clip on the rod inside the nose of the MC to keep the rod from pulling out...that's the stop? If that's right then he will have to fab up a bump stop on the toe board.

Thanks Carl!

Mike B smile

Joined: Jun 2019
Posts: 151
B
'Bolter
Originally Posted by Mike B
So if I understand right, the AD trucks have a washer/clip on the rod inside the nose of the MC to keep the rod from pulling out...that's the stop? If that's right then he will have to fab up a bump stop on the toe board.

Thanks Carl!

Mike B smile

After doing some digging, I found out a couple things about the original AD master cylinders. First, the piston end of the push rod is ball shaped (not just round on the end), and 2.) the end of the master cylinder has a sheet metal plate with a hole in the middle that the push rod passes through. This plate is held in place by a wire spring clip. So, the push rod is captured inside the master cylinder housing between the piston on the end plate. This set up is what makes it possible to adjust the position of the brake lever by shortening/lengthening the push rod at the clevis.

As 52Carl pointed out, the Corvette master cylinder does not have a captive push rod. Because of this, shortening/lengthening the push rod does nothing to change the position of the brake pedal lever. The lever goes all the way up until it hits the brake light switch no matter what. You still have to adjust the length of the push rod to take up the lost motion at the master cylinder so that your pedal works sooner than later.

At this point I think the easiest solution to my situation would be to buy one of those rubber bumpers that goes on the clutch arm, then cut one end down so that it sets the arm at the required 13/16" from the toeboard.

I thank and appreciate everyone's input on this thread.


Brian

'51 Chevy 3604 Project
'28 Chevy LO basket case
'83 GMC Sierra 4x4
Joined: Jul 2000
Posts: 1,538
G
Insomniac
The 'required 13/16"' applies to the original m/c. This no longer applies now that you have changed it. I'm much closer than that.


Gord
----
1954 1/2 ton 235 4 speed
Joined: Mar 2010
Posts: 10,155
5
'Bolter
The 13/16" gap sure would work nice for the original brake light switch.

Joined: Mar 2004
Posts: 5,538
M
'Bolter
Originally Posted by Brian Wise
After doing some digging, I found out a couple things about the original AD master cylinders. First, the piston end of the push rod is ball shaped (not just round on the end), and 2.) the end of the master cylinder has a sheet metal plate with a hole in the middle that the push rod passes through. This plate is held in place by a wire spring clip. So, the push rod is captured inside the master cylinder housing between the piston on the end plate. This set up is what makes it possible to adjust the position of the brake lever by shortening/lengthening the push rod at the clevis.

Your description is what I was thinking about...we're on the same page now!

Whatever you make for a bump stop it needs to be robust enough to never fail allowing the rod to suddenly pull out...rubber could squeeze out or collapse over time.

Mike B smile

Joined: Jun 2019
Posts: 151
B
'Bolter
Originally Posted by Gord&Fran
The 'required 13/16"' applies to the original m/c. This no longer applies now that you have changed it. I'm much closer than that.

The 13/16" dimension, per the Factory Assembly Manual, is from the pedal arm to the toe board. I'm guessing that is the recommended clearance for the pedal arm to properly operate the brake light switch?


Brian

'51 Chevy 3604 Project
'28 Chevy LO basket case
'83 GMC Sierra 4x4
Joined: Jul 2000
Posts: 1,538
G
Insomniac
I use the factory switch. Seems to work OK. ohwell


Gord
----
1954 1/2 ton 235 4 speed
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