The Stovebolt Site Forums The Shop Area General Truck Talk Yet another brake pedal travel question

I've been using the Search function till my eyes cross but cannot find an answer to my dilemma.

During my rebuild project I upgraded to a dual-circuit master cylinder. The pedal arms got new bushings, new return springs, etc. Everything works fine, and I've even managed to test drive the bare chassis a couple times (the cab is still not on the chassis, but I'm getting close to putting it back on). So here's the question: is there supposed to be some kind of a clip, etc., that prevents the master cylinder piston rod from being pulled away from the piston? The brake pedal spring pulls the arm all the way till the pedal hits the hypothetical cab floor. There's nothing to prevent it from pulling the piston rod almost out of the rubber boot. Because of this situation, adjusting the length of the piston rod is meaningless at this point. Something needs to keep the piston rod up tight to the piston to eliminate all of that unwanted travel. Thoughts anyone?

It might help if we knew what year and model truck you're working on...

Mike B

Oh crap! Sorry about that! It's a 1951 3604.

I had to do some digging but managed to find one photo of my set up. Note that the brake pedal arm is straight up in the air (vs. the clutch pedal arm).

You need the cab on before you can make the final adjustments, the brake pedal arm has a rubber bumper that stops the pedal return travel when it hits the toe board.

Mike B

Hey Mike,
I believe that you are thinking of the bumper for the clutch pedal. The brake pedal lever does not take a bumper. I bumper on the brake pedal lever could very well cause the the master cylinder to malfunction. (I am sure that you already know this. )

The final placement of the brake pedal is governed by two things.
1) The piston being held in the forward position by the internal piston spring, and supplemented by the brake lever return spring.
2) The length of the rod coming from the master cylinder, connecting to the brake pedal lever. (Yours appears to be way too long.)

OOPS, you're right Carl, got my pedals confused, but I still say you can't adjust the brake pedal without the cab in place...

Mike B

I hear you guys on the need to have the cab in place to do the final adjustments. I totally get that. The shop assembly manual illustrates the method for adjusting the brake pedal to achieve a 13/16" gap between the lever and the toeboard. The brake lever spring is constantly trying to force the lever into the cab, so the only way that adjusting the length of the rod at the master cylinder can be effective is that the rod must be anchored to the master cylinder somehow. I've noticed that images of replacement master cylinders (same style as the OEM cylinders) usually come with the rod sticking out of the rubber boot. Is this rod actually attached to the end of the piston?

As it sits in the photo I posted, if I move the brake pedal lever forward until the brake rod touches the face of the piston at the master cylinder, the lever will be even with the clutch pedal lever. At that point there is no lost motion in the linkage. This just happens to be how I have the rod length adjusted for now.

Thinking about how the brake pedal operates, in my mind the only thing that is going to stop the brake lever travelling towards the cab will be the arm on the brake light switch. If that is the case, it won't be long before it's beat to death. Or at the least it will beat its way into the toeboard.

In my case, I'm using a 1960s Corvette master cylinder (from NAPA) as recommended by the upgrade kit manufacturer. It did not come with the rod. The master cylinder upgrade kit came with the rod which has a round end that contacts the piston.

To help illustrate what I'm working with, I've cropped a catalog cut showing a master cylinder set up similar to mine. It clearly shows the rod with the round end that butts up against the piston. There is nothing to hold it in place, therefore the brake pedal spring pulls it away from the master cylinder as far as the lever will travel. So what's the secret to keeping the pedal lever from always wanting to go through the toeboard?

Doesn't the master cylinder return pressure push the brake pedal back into position? I may be looking at this wrong but shouldn't the brake lever return spring need a stop block or be moved to where it will hold the pedal in the correct position?

I would find the proper pedal height and mount the return spring in that position so the rod stays in contact with the master cylinder. The spring should maintain tension on the pedal assembly through it's range of motion and return it to the starting position. Remember you are not going to be able to use the factory position for the return spring now that you have changed to a dual master cylinder. It will probably come down to trial and error to find the correct location for the spring.

The rod is not held in place by any dedicated fastener. Pressure from the pedal mechanism keeps it in place.

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.

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.

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.

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.

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.

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.

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.

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.

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

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

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.

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" gap sure would work nice for the original brake light switch.

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

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?

I use the factory switch. Seems to work OK.