Bent Pushrod from Brake Pedal to M/C

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Ken Tuvman
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Bent Pushrod from Brake Pedal to M/C

#1 Post by Ken Tuvman » Fri Sep 23, 2022 9:04 am

My barn find 356 sat under a cover for 20 years -over the past 3 years it’s been my rolling restoration project - one of the persistent issues was the brake pedal not returning when depressed. Knowing the pedal assembly was pretty dirty and gritty, I removed it, sandblasted all components, primed and painted the pedal assembly, and ordered a new brake return spring and pedal stops. I used a combination of regular grease and moly grease on the rod that moves the clutch pedal to cable and the pivot on brake pedal. After reassembly, had the same problem with brake pedal not returning. Knowing pedal assembly rebuild was not the issue, I started focusing on the pushrod that attaches to brake pedal and goes into the master cylinder. I kept looking at it trying to figure out what was causing it to bind when depressed - brakes work but pedal not returning.

After a day or two, an idea popped into my head to put the pushrod in my drill press and when I did it wobbled like crazy, confirming that it is indeed bent. The seller told me the car was raced so maybe hard braking on the track? Maybe?

I reached out to Paul Domitrovic at Stoddard - Paul is very familiar with the parts they remanufacture and sell. I asked him, "how does a m/c pushrod get bent"- he says it could have been an accident or a frozen piston in the previous m/c. Didn't ask Paul but anyone know how a frozen piston in m/c could bend the rod?

Anyone ever experience anything like this? Hoping the new pushrod I ordered solves the problem. Planning to write a story for registry magazine so any words of wisdom / experiences to share are greatly appreciated > Ken
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Re: Bent Pushrod from Brake Pedal to M/C

#2 Post by Don Gale » Fri Sep 23, 2022 9:52 am

kL/r
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Re: Bent Pushrod from Brake Pedal to M/C

#3 Post by Martin Benade » Fri Sep 23, 2022 9:54 am

Maybe wrenches were pushed hard against the bracket while adjusting it sometime in the past.
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Re: Bent Pushrod from Brake Pedal to M/C

#4 Post by Wes Bender » Fri Sep 23, 2022 10:43 am

Couldn't you just straighten it?
Some days it's just not worth chewing through the restraints.....

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Re: Bent Pushrod from Brake Pedal to M/C

#5 Post by Phil Planck » Fri Sep 23, 2022 11:43 am

Critical safety part. I would replace.
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Re: Bent Pushrod from Brake Pedal to M/C

#6 Post by Ken Tuvman » Fri Sep 23, 2022 11:46 am

Phil Planck wrote:
Fri Sep 23, 2022 11:43 am
Critical safety part. I would replace.
I'm with you Phil - new one on the way - tried rotating existing rod at different angles and still no good. Paul also suggested getting a new one - just wondering how this got bent. I've got a video but this platform won't enable me to upload it - it is posted on FB 356 Registry page.
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Re: Bent Pushrod from Brake Pedal to M/C

#7 Post by Dick Weiss » Fri Sep 23, 2022 5:33 pm

HI Ken.

Hard to believe the brake pushrod got bent--, by how much? Photo?

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Re: Bent Pushrod from Brake Pedal to M/C

#8 Post by Ken Tuvman » Fri Sep 23, 2022 8:54 pm

Hi Dick - I’ll email you a very short video
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Re: Bent Pushrod from Brake Pedal to M/C

#9 Post by Don Gale » Fri Sep 23, 2022 11:54 pm

Just for grins, I modelled one from my bonepile to see what's going on. Mine is bent also, at the hex nut, and has about +/- 1/4" wobble at the tip when chucked in a lathe. The culprit is the offset one-sided clevis at the pedal lever which imparts a bending moment on the rod. I applied a 75 lb force on the brake pedal which translates to 400 lb at the tip of the rod where it inserts in the m/c socket. You can see the 2 most likely places for it to bend are at the hex jam nut and at the reduced step at the boot collar. I can't be sure what alloy Porsche used so I guessed with a medium carbon steel, and the results are right at the yield point.
pushrod.jpg
pushrod_model.jpg
pushrod_stress.jpg
Edit: I tweaked the model and included the threads next to the jam nut which reduces the cross-section. I also refined the boundary conditions, keeping the same 75 lb applied force at the brake pedal, which is not unrealistic. The stresses shot thru the roof. I must conclude that your bent rod is not an isolated case and perhaps there are others on the road with slightly bent rods that have never experienced a frozen m/c piston. When I put my coupe back on the road, I'm going to machine a 4140 rod and eliminate the hole for the pin at the boot collar and replace it with a set screw and heat treat and normalize it.
Last edited by Don Gale on Sat Sep 24, 2022 5:34 pm, edited 1 time in total.
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former 1978 Intermeccanica Speedster w/'68 912
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Re: Bent Pushrod from Brake Pedal to M/C

#10 Post by Martin Benade » Sat Sep 24, 2022 1:15 am

The clevis flexes, causing the rod to bend?
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Re: Bent Pushrod from Brake Pedal to M/C

#11 Post by Ken Tuvman » Sat Sep 24, 2022 8:35 am

Don - thanks for taking time to test the rod and explore, explain, and share - the images are off the charts amazing!
The drill press test gave me a visual where I believe problem pedal not returning lies but this is great!

Thanks so much!
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