Stovebolt.com 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.