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Old 07-07-2018, 03:21 PM   #1
mh_merc
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Default Any brake geniuses out there?

It all started with a pair of leaking wheel cylinders on my '47 Marmon-Herrington/Merc woodie, equipped with a Chevy 350 and Hydrovac booster. After battling with a bunch of repro crap and several adjustments and system bleeding, we ended up with 4 good cylinders, properly fitting shoes, new rubber hoses, new master cylinder and a firm pedal with the right amount of free play.
It all worked fine on the way to our local C & C today, but on the way home - about 5 miles - the pedal got higher and higher. Finally back home, there was about zero free play, the brake lights were stuck on, and I couldn't budge the car with a push. After about an hour, I checked it again and everything was back to normal.
I suspect something blocked or stuck in the booster, since it's the only component we didn't replace...any ideas?
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Old 07-07-2018, 03:44 PM   #2
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Default Re: Any brake geniuses out there?

Not answering as a brake genius, but your symptoms are like the pushrod to the master is too long. Can you verify that there is 1/16" to 1/8" free play before the rod contacts the piston in the cylinder?
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Old 07-07-2018, 04:11 PM   #3
Tony, NY
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Default Re: Any brake geniuses out there?

Not a genius either, but sounds like a heat problem due to brake shoe adjustment or a brake line too close to exhaust somewhere.
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Old 07-07-2018, 04:44 PM   #4
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Default Re: Any brake geniuses out there?

Did you ever pinch the rubber lines that will cause that. MC not going back all the way.
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Old 07-07-2018, 05:00 PM   #5
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Default Re: Any brake geniuses out there?

I concur with not enough free play on the booster push rod.
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Old 07-07-2018, 05:00 PM   #6
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Default Re: Any brake geniuses out there?

No1 suspect is the pushrod length, or the recuperation port being blocked for some other reason.

If out on the road and this is a problem on a trip, the easiest thing to do to alleviate the problem is to just crack a bleed nipple. That should get you down the road until you need to do it again. Spacing the master cylinder off the booster with a couple of temporary washers is also a good get you home fix.

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Old 07-07-2018, 05:03 PM   #7
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Default Re: Any brake geniuses out there?

I agree- pushrod length.
It has happened to me several times.
Shorten the rod 1/8 inch and try it again.
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Old 07-07-2018, 05:17 PM   #8
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Default Re: Any brake geniuses out there?

Push Rod length
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Old 07-07-2018, 05:19 PM   #9
mh_merc
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Default Re: Any brake geniuses out there?

Thanks for the quick replies! The majority consensus seems to be pushrod length...would this be something that gets worse as the brakes heat up?
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Old 07-07-2018, 05:28 PM   #10
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Default Re: Any brake geniuses out there?

No, the pushrod length has to do with a port in the MC that allows fluid back to the reservoir when you let off the brakes. IF the piston doesn't come back far enough, you get more and more fluid in the system out to the shoes until it is locked up.
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Old 07-07-2018, 05:50 PM   #11
mh_merc
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Default Re: Any brake geniuses out there?

Thanks - makes sense to me. I'll get on the adjustment tomorrow. But how did they unlock after sitting for a while?

Last edited by mh_merc; 07-07-2018 at 05:55 PM. Reason: Added a question
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Old 07-07-2018, 06:19 PM   #12
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Default Re: Any brake geniuses out there?

The fluid heats up/expands and applies pressure to the brakes. When the fluid cools the pressure releases. Allowing the MC piston to retract all the way prevents the pressure build up.
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Old 07-07-2018, 06:49 PM   #13
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Default Re: Any brake geniuses out there?

More good info...thanks.
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Old 07-07-2018, 07:21 PM   #14
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Default Re: Any brake geniuses out there?

I'm having same problem and as 40cpe says it's the heat that starts the problem. I've been adjusting the MC push rod along with the brakes shoes to try and find a good balance for the fluid temp. One turn on the pushrod can make the difference. Check your brake lights after every adjustment.
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Old 07-07-2018, 08:23 PM   #15
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Default Re: Any brake geniuses out there?

adjust the push rod so that you can "rattle" it around in the master cylinder, also if you set up the brake shoe adjustment too tight the heat caused by driving will expand the friction lining enough to cause a drag which could be a
problem also. I always leave the shoes a little loose and then drive the car to heat up the shoes then do a final adjustment
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Old 07-08-2018, 06:51 AM   #16
51 MERC-CT
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Default Re: Any brake geniuses out there?

Quote:
Originally Posted by alanwoodieman View Post
adjust the push rod so that you can "rattle" it around in the master cylinder, also if you set up the brake shoe adjustment too tight the heat caused by driving will expand the friction lining enough to cause a drag which could be a
problem also. I always leave the shoes a little loose and then drive the car to heat up the shoes then do a final adjustment
And what about the drums that expand in diameter when hot?
Wouldn't that counteract any perceived expansion of the friction lining?
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Old 07-08-2018, 07:10 AM   #17
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Default Re: Any brake geniuses out there?

Push rod or a residual valve sticking
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Old 07-08-2018, 08:18 AM   #18
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Default Re: Any brake geniuses out there?

Quote:
Originally Posted by mh_merc View Post
It all started with a pair of leaking wheel cylinders on my '47 Marmon-Herrington/Merc woodie, equipped with a Chevy 350 and Hydrovac booster.
Maybe a it needs a " Chevy 350 " compatible push rod
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Old 07-08-2018, 08:20 AM   #19
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Default Re: Any brake geniuses out there?

Quote:
Originally Posted by 34PKUP View Post
Maybe a it needs a " Chevy 350 " compatible push rod
Funny
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Old 07-08-2018, 08:51 AM   #20
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Default Re: Any brake geniuses out there?

optimum word expand, do drums expand into the shoes or away from the shoes???
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