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Re: Brakes drag after 10 minutes.
Lugnutz #1357142 Sat Apr 25 2020 11:25 PM
Joined: Apr 2005
Posts: 8,664
Grease Monkey, Moderator General Truck Talk & Greasy Spoon
As Jerry just mentioned, you could have a bad M/C, I too have had that issue.


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)
‘39 Dodge Businessmans Coupe
USAF 1965-69 Weather Observation Tech (got paid to look at the clouds)


"I fought the law and the law won" now I are a retired one!
Support those brave men/women who stand the "Thin Blue Line"! Hug a cop!

Re: Brakes drag after 10 minutes.
Lugnutz #1357146 Sat Apr 25 2020 11:35 PM
Joined: Sep 2011
Posts: 1,962
L
Lugnutz Offline OP
Shop Shark
Justhorsenround,
I backed the pushrod up 1/4” and went for a drive. About lap 3 around the block I began to panic because I sure I was gonna have to eat humble pie. But by lap 6 I began to feel the drag again. You had me sweating big time. The push rod is sloppy loose and the brakes are tight as a tick right now.
So I’m back to looking at a problem with either the MC or something in the front lines that restricts return flow to the MC.
I even ran the engine at idle in the driveway until it got hot under the hood. The exhaust downpipe was 250* but the brake hard lines were 150* at the level of the frame where they are closest to the downpipe. I pumped the pedal repeatedly but could not get it to tighten up like it does when driving.

Attached Files
Last edited by Lugnutz; Tue Apr 28 2020 01:33 PM.
Re: Brakes drag after 10 minutes.
Lugnutz #1357151 Sat Apr 25 2020 11:42 PM
Joined: Aug 2000
Posts: 1,372
D
Shop Shark
Maybe I missed you saying, but are you using a 2# residual valve on the fronts? If so, maybe it is failing? Or, there is a 10# one there?

Dennis


40 Chevy 1/2 ton
Re: Brakes drag after 10 minutes.
Lugnutz #1357152 Sat Apr 25 2020 11:51 PM
Joined: Mar 2019
Posts: 736
G
Shop Shark
Do you have a known good master cylinder? If not, buy one, it wont goto waste. If you eliminated the rear, and cracked the front bleeder like i mentioned a while ago, and the brakes loosened. Bingo.

Re: Brakes drag after 10 minutes.
Lugnutz #1357172 Sun Apr 26 2020 01:23 AM
Joined: Sep 2011
Posts: 1,962
L
Lugnutz Offline OP
Shop Shark
I just had a thought. The current set up consists of a 1970’s Corvette style master cylinder. I’ve seen lots of cars and trucks use these when they upgrade to disc brakes. The MC piston has a place for the pedal pushrod for applications when a booster is not used. With a booster, a short piece of steel rod is inserted in the back of the MC piston to fill that hole and provide a place for the booster pushrod to push on to move the MC piston. If the steel rod is too long, then the master cylinder would never be able to fully retract to the home position but on the bench, the MC would work normally during the bench bleed. What do you think?
You can see the steel rod in question in this picture. Am I crazy?

Attached Files
B9B92FEB-B2B3-44F2-8129-33EBF6E292A1.jpeg (106.91 KB, 120 downloads)
Re: Brakes drag after 10 minutes.
Lugnutz #1357181 Sun Apr 26 2020 02:12 AM
Joined: Feb 2004
Posts: 20,371
H
Boltergeist
Get a piece of cold rolled round stock the same diameter as the spacer, and cut off a piece about 1/8" shorter, and try a test drive with the short piece installed. You won't be making any sort of permanent modification that can't be reversed by simply reinstalling the original part.
Jerry


The murder victim was drowned in a bathtub full of Rice Krispies and milk.
The coroner blamed the crime on a cereal killer!

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
Re: Brakes drag after 10 minutes.
Lugnutz #1357202 Sun Apr 26 2020 03:05 AM
Joined: Sep 2011
Posts: 1,962
L
Lugnutz Offline OP
Shop Shark
Ladies and gentlemen! I am happy to bring this story to an end.

The problem was the intermediate pushrod between the booster and the MC. It was approximately 1/8” too long. This was not noticeable when bolting the MC to the booster during assembly. However, once the MC was bolted to the booster, the piston in the MC was pushed in 1/8” and no longer at the “home” position. That means the MC piston was never able to properly release pressure from the front lines. So pressure in the front brakes would just keep building.

I removed the intermediate rod and functionally shortened it by drilling out the center about 1/8” so the nipple on the booster rod had more room. Put it all back together. Test drive and NO MORE DRAGGING OF THE BRAKES!

Time for a HAPPY DANCE!

Attached Files
31C85DEF-61E8-49B9-8316-1067B2280BD6.jpeg (125.76 KB, 107 downloads)
5F87245D-1D20-4798-9AE4-50CC529D288A.jpeg (191.12 KB, 111 downloads)
2CA6A5BF-040B-42DB-BC08-7E9D321015F2.jpeg (266.25 KB, 105 downloads)
Re: Brakes drag after 10 minutes.
Lugnutz #1357209 Sun Apr 26 2020 03:52 AM
Joined: Feb 2004
Posts: 20,371
H
Boltergeist
Look at your photo of the booster- - - - -that looks a lot like a bolt and a lock nut where the plunger pushes the master cylinder. Dimpling the adapter accomplished the same thing, but in case you need to make any further adjustments, you can fine tune it in either direction without doing any more drilling.
Jerry


The murder victim was drowned in a bathtub full of Rice Krispies and milk.
The coroner blamed the crime on a cereal killer!

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
Re: Brakes drag after 10 minutes.
Hotrod Lincoln #1357212 Sun Apr 26 2020 04:15 AM
Joined: Sep 2011
Posts: 1,962
L
Lugnutz Offline OP
Shop Shark
Originally Posted by Hotrod Lincoln
Look at your photo of the booster- - - - -that looks a lot like a bolt and a lock nut where the plunger pushes the master cylinder. Dimpling the adapter accomplished the same thing, but in case you need to make any further adjustments, you can fine tune it in either direction without doing any more drilling.
Jerry
Yes, I tried to adjust that first while my wife patiently held the MC so it wouldn’t bend the brake lines. No matter what I did, it didn’t seem to change the booster rod tip length. So I resorted to drilling the spacer for the same result.

Re: Brakes drag after 10 minutes.
Lugnutz #1357231 Sun Apr 26 2020 12:39 PM
Joined: Apr 2005
Posts: 8,664
Grease Monkey, Moderator General Truck Talk & Greasy Spoon
Don’t you just love it when a plan comes together. Good job.🛠


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)
‘39 Dodge Businessmans Coupe
USAF 1965-69 Weather Observation Tech (got paid to look at the clouds)


"I fought the law and the law won" now I are a retired one!
Support those brave men/women who stand the "Thin Blue Line"! Hug a cop!

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