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Joined: Apr 2005
Posts: 11,129
Grease Monkey, Moderator General Truck Talk & Greasy Spoon
The rod is not held in place by any dedicated fastener. Pressure from the pedal mechanism keeps it in place.


Martin
'62 Chevy C-10 Stepside Shortbed (Restomod in progress)
'47 Chevy 3100 5 Window (long term project)
‘65 Chevy Biscayne 4dr 230 I-6 one owner (I’m #2) “Emily”
‘39 Dodge Businessmans Coupe “Clarence”



"I fought the law and the law won" now I are a retired one!
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Joined: Jun 2019
Posts: 152
B
'Bolter
I've gone back and watched a couple of the Brothers Trucks videos on YT about upgrading to a dual master cylinder. Some of those videos are such bad quality as to be difficult to watch. The one video where he installs the new kit under the truck, though, did answer one question for me. When he removed the original single circuit cylinder, the actuating rod came out with the cylinder, not seperately. That tells me that it is attached to the piston in some fashion (I don't have my old one or I'd take it apart and look). It makes sense to me that the brake spring is there to help the small master cylinder get the pedal back to the correct position.

Now, when he installed the new dual circuit master cylinder, he first installed the new actuating rod to the brake pedal arm, then installed the new cylinder, slipping the rod into the end of the housing. He then adjusted the length of the actuating rod to get it close to the piston, but leaving just a little free play (for heat expansion he says). At no time did he do anything with the original brake pedal spring. I have to imagine that the spring has that pedal arm up tight against the brake light switch.

I'd sure like to see pictures of other trucks that have had this conversion done to see if the owners changes the pedal spring in some way (either its size, or position), or...etc. At this point what I'm thinking of doing is attaching a 13/16" thick rubber block to the toeboard just below the brake light switch to give the brake pedal arm something to stop against, or adding a rubber stop to the arm similar to the clutch pedal.


Brian

'51 Chevy 3604 Project
'28 Chevy LO basket case
'83 GMC Sierra 4x4
Joined: May 2015
Posts: 5,934
Housekeeping (Moderator) Making a Stovebolt Bed & Paint and Body Shop Forums
I was thinking that the original pedal return spring might be too strong for the new master cylinder. You might try without it or maybe shorten the extra link so it is looser when the pedal is almost retracted. Or your toeboard bumper idea might be good as well.


Kevin
Newest Project - 51 Chevy 3100 work truck. Photos [flickr.com]
#2 - '29 Ford pickup restored from the ground up.
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5
'Bolter
In the original master cylinder, the rod is attached to, and captivated by a disk (a washer more or less). This rod-captivating disk is held in place by a lip in the forward end of the casting. The rear face of the disk pushes the piston to the rear when the pedal is depressed.
The original brake return spring pulls the brake lever, thus the rod forward, ensuring that the internal piston spring (which is located behind the piston) can push the piston forward unobstructed.
I have no idea how your new master cylinder could ever work for this application as you have the linkage set up.
You need to find more info on how your master cylinder is supposed to be installed.

Last edited by 52Carl; Wed Feb 16 2022 12:17 AM.
Joined: Sep 2020
Posts: 102
H
'Bolter
I feel the original posters concern, an important factor is the chosen master cylinder, it is critical the depth of the pushrod into the master cyl piston is deep (like over 1”).
If you used a C3 corvette master (68-70+-) for manual brakes it should be very deep, this depth keeps the pushrod safely captive in the master.
If you don’t have this deep piston well for the pushrod? ….you need it…get another master.


Hank: 46 Chev 1/2ton shortbed
2018 Miata RF
Joined: Jul 2000
Posts: 1,613
G
Insomniac
Here's CPP's instructions. Seems similar to what you are doing...

https://www.classicperform.com/Instructions/PDF/4754MCA.pdf

In my under floor master cyl setup, I have a power brake booster, so the pushrod cannot fall out.


Gord
----
1954 1/2 ton 235 4 speed
Joined: Jun 2019
Posts: 152
B
'Bolter
Originally Posted by Hanks custodian
I feel the original posters concern, an important factor is the chosen master cylinder, it is critical the depth of the pushrod into the master cyl piston is deep (like over 1”).
If you used a C3 corvette master (68-70+-) for manual brakes it should be very deep, this depth keeps the pushrod safely captive in the master.
If you don’t have this deep piston well for the pushrod? ….you need it…get another master.

Yes, I chose a C3 Vette master, which was the recommended master for this application. The push rod does goes in quite a ways. There is no danger of the rod falling out of the end of the master. But that's not the issue that I'm trying to figure out.

The CPP instructions that Gord&Fran linked to is just like my installation.

Here again is where I'm baffled: since the master cylinder end of the push rod is not captured at that end, the pedal return spring is allowed to pull on the bottom of the arm, swinging it upward as far as it will go. The only thing stopping the arm will be the brake light switch arm. If that weren't there, the pedal arm would simply bash into the toeboard. Eventually, taking your foot off of the brake pedal time after time will beat up the switch arm and the paint on the toeboard. In this scenario, shortening the length of the push rod would do nothing except move the end of the rod further away from the piston (then you'd have to push the pedal further to get any braking).

I can't be the only guy on this forum that has installed a dual-circuit master cylinder conversion (without the power booster) in their AD truck. I'd just like to know what those other folks did that maybe I didn't do, need to do differently or haven't done yet. Depending on what that is, it might be easier to do the work before I put the cab back on the frame, and before I paint the cab.


Brian

'51 Chevy 3604 Project
'28 Chevy LO basket case
'83 GMC Sierra 4x4
Joined: May 2015
Posts: 5,934
Housekeeping (Moderator) Making a Stovebolt Bed & Paint and Body Shop Forums
Originally Posted by Brian Wise
The only thing stopping the arm will be the brake light switch arm. If that weren't there, the pedal arm would simply bash into the toeboard. Eventually, taking your foot off of the brake pedal time after time will beat up the switch arm and the paint on the toeboard.
It'll eventually wear a hole in the toeboard. My cab was sagged enough that the brake switch arm wore into the toeboard, which was another hole I needed to weld up.


Kevin
Newest Project - 51 Chevy 3100 work truck. Photos [flickr.com]
#2 - '29 Ford pickup restored from the ground up.
First car '29 Ford Special Coupe
Busting rust since the mid-60's
Joined: Mar 2007
Posts: 1,820
Crusty Old Sarge
Once your cab is in place you could fabricate a bump stop. Of course you would need to account for the movement of the brake light switch and travel required to engage it. A simple rubber button or pad should work fine.


Craig

Come, Bleed or Blister something has got to give!!!
'59 Apache 31, 327 V8 (0.030 over), Muncie M20 4 Speed, GM 10 Bolt Rear... long term project (30 years and counting)
Joined: Mar 2004
Posts: 5,618
M
'Bolter
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

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