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09-02-2019, 05:33 PM
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#1 |
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Senior Member
 
Join Date: Oct 2012
Location: Lexington, Ky
Posts: 2,113
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Brake Problem
My 72 has power brakes, 350/auto. Noticed that some times, while sitting in traffic with brakes applied, the pedal slowlystarts to go to the floor. I have to lift and reapply then they're good, with full pedal.
It seems to happen sporadically, only once every 3 or 4 days. And seems to happen when I sit for full minute or longer. Couple of times truck acutally started to move. Any ideas or theories on what to check? Master cylinder has plenty of fluid. Cant see any leaks at wheels. Vacuum leak, or ???
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Larry 84 Chevy SWB, "Roy", dark blue orig paint, 6 cyl auto, PS, 88,000 miles. 72 Highlander LWB "Olive" med olive over lt yellow, air, 77,000 miles (sold) "It's a sorry man who can't find his wife a good job" |
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09-02-2019, 05:48 PM
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#2 |
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Post Whore
Join Date: Feb 2016
Location: Portland Oregon
Posts: 10,804
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Re: Brake Problem
First thing that comes to mind is the fliud is leaking by the master cylinder seals internally. Usually requires a master cylinder replacement.
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Thanks to Bob and Jeanie and everyone else at Superior Performance for all their great help.  RIP Bob Parks. 1967 Burban the WMB,1991 S(stink)-10 Blazer,1969 GTO, 1970 Javelin, 1952 F2 Ford 4X4, 29 Model A, 72 Firebird. 85 Alfa Romeo If it breaks I didn't want it in the first place The WMB repair thread http://67-72chevytrucks.com/vboard/s...d.php?t=698377 |
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09-02-2019, 07:18 PM
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#3 |
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Registered User
Join Date: Aug 2019
Location: Harrisburg, Pennsylvania
Posts: 66
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Re: Brake Problem
Agree with HO455. Sounds like the master is leaking down internally
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09-02-2019, 09:30 PM
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#4 |
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Registered User
Join Date: Jan 2009
Location: Las Cruces New Mexico
Posts: 219
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Re: Brake Problem
I just experienced the same symptoms. New master cylinder fixed it.
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Justin 1970 Blazer in progress |
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09-02-2019, 10:07 PM
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#5 |
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Registered User
Join Date: Apr 2011
Location: Fairview Park, Ohio
Posts: 1,030
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Re: Brake Problem
Yep. Replace master
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My build page http://67-72chevytrucks.com/vboard/s...d.php?t=764662 My cabinet building site http://www.cbcabinets.com |
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09-02-2019, 11:18 PM
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#6 |
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Registered User
Join Date: Feb 2019
Location: So Cal, CA
Posts: 108
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Re: Brake Problem
Master cylinder
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1971 C20 Longhorn 350/350 "Conquistador" |
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09-03-2019, 12:02 AM
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#7 |
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Registered User
Join Date: Feb 2019
Location: Nor Cal
Posts: 822
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Re: Brake Problem
Might be a Master Cylinder..
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1968 C20 Fleetside 396 4sp PS PB AC CST . https://www.youtube.com/watch?v=RWK8GgWD4uA |
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09-03-2019, 10:20 AM
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#8 |
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Registered User
Join Date: Apr 2014
Location: Azle,Texas
Posts: 2,248
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Re: Brake Problem
Yep. Master cylinder. If you don’t want to replace it for whatever reason, there are rebuild kits available. In my broke teenage years (the 80’s) I rebuilt at least 3 of them. Basically the process is full disassembly, thoroughly clean it up, hone the cylinder out with a drill and one of those spring loaded tools that have 3 honing stones attached, new plunger and finally new seals. The kit was about 1/4 the price of a rebuilt master cylinder back then.
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Brian 1972 C10, "Loyd", LWB to SWB, 5.3, L83/6L80e, 4:11 Tru Trac, Air Ride, VA, DD, 20" Coys, 4 wheel disc, A quick LS swap turned into a 6 year frame off resto-mod. |
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09-03-2019, 10:42 AM
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#9 |
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Registered User
Join Date: Jan 2009
Location: Las Cruces New Mexico
Posts: 219
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Re: Brake Problem
Rebuilt master cylinder was $27 at O’Reilly’s. Difficult to justify doing that much work unless you just want to say you did.
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Justin 1970 Blazer in progress |
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09-03-2019, 11:34 AM
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#10 |
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Registered User
Join Date: Jul 2015
Location: 2nd left past the stump on a dirt road.
Posts: 2,629
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Re: Brake Problem
When the Master leaks internally like some say, It will dump fluid into the booster and ruin that also.
For the low cost involved I would just replace it. Do a leak test on the booster also with a spray bottle of soapy water. You may save yourself from a bunch of grief now doing this. |
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09-03-2019, 11:48 AM
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#11 |
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Senior Member
 Join Date: Aug 2019
Location: Cheraw
Posts: 1,305
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Re: Brake Problem
If you replace the Master cylinder,don't forget to flush all of the old brake fluid from the brake lines and if the rubber hoses are old replace them now to avoid a bigger problem latter on. Brake fluid gets old and turns color from contamination.
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09-03-2019, 04:43 PM
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#12 |
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Senior Member
Join Date: Oct 2012
Location: Lexington, Ky
Posts: 2,113
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Re: Brake Problem
Thank you all for the replies. I'm replacing the master cylinder and the booster too. Been looking at YouTube videos on bench bleed and such. You guys are the best. I appreciate you all putting me on the right diagnosis at the start. Will post when I get it done.
__________________
Larry 84 Chevy SWB, "Roy", dark blue orig paint, 6 cyl auto, PS, 88,000 miles. 72 Highlander LWB "Olive" med olive over lt yellow, air, 77,000 miles (sold) "It's a sorry man who can't find his wife a good job" |
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09-19-2019, 09:41 AM
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#13 |
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Senior Member
Join Date: Oct 2012
Location: Lexington, Ky
Posts: 2,113
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Re: Brake Problem
Got new master cylinder (Classic Parts) and booster (CPP) installed.
Again, Thanks to all who replied.
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Larry 84 Chevy SWB, "Roy", dark blue orig paint, 6 cyl auto, PS, 88,000 miles. 72 Highlander LWB "Olive" med olive over lt yellow, air, 77,000 miles (sold) "It's a sorry man who can't find his wife a good job" |
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