#1  
Old 12-04-2020, 05:41 PM
scott70's Avatar
scott70 scott70 is offline
Ultimate Warrior
 
Join Date: May 2002
Location: maine
Posts: 2,220
Default 70 gto brakes

My car is a factory power disc brake car. I had an issue of if you hit the brakes semi hard to hard it would lock up the rears at speed. I replaced the valve thats right underneath the master cylinder and the smaller brass valve, the one thats on the driver side in the rear, its right where the line turns to the rear cross member. Now if your going real slow down the driveway and touch the brakes it locks up the rears but if your driving at a higher speed the brakes seem to act normal . Just trying to figure out the slow speed rear brake lock up. Thanks

__________________
72 lemans,455 e-head, UD 255/263 solid flat,3.73 gears,,,10" 4400 converter,, 6.68 at 101.8 mph,,1.44 60 ft.2007
(cam 271/278 roller)9"CC.4.11gear 6.41 at 106.32 mph 1.42 60 ft.(2009) SOLD,SOLD
1970 GTO 455 4 speed #matching,, 3.31 posi.Stock manifolds. # 64 heads.A factory mint tuquoise ,69' judge stripe car. 8.64 @ 87.3 mph on slippery street tires.Bad 2.25 60ft.Owned since 86'
  #2  
Old 12-04-2020, 07:17 PM
Schurkey Schurkey is offline
Ultimate Warrior
 
Join Date: Aug 2002
Location: The Seasonally Frozen Wastelands
Posts: 5,891
Default

Rear shoes contaminated? Leaking brake fluid or leaking axle lube are two common problems. My experience is that it's worst when the brakes are cold.

Drum finish acceptable? Rough or scored drums may make this worse.

Shoes adjusted properly? If one side is looser than the other, can cause the tight side to lock up.

Master cylinder bore/wheel cylinder bore ratio correct?

Front brakes weak (example: Seized calipers) leading to over-use of rear brakes?

Are the replacement valves that you installed OEM-quality, or aftermarket junk? I've heard a lot of problems with aftermarket combination or proportioning valves--air leaks into the system, fluid leaks out of the system, internal parts seized and non-functional, etc. But the brass is nice 'n' shiny!

  #3  
Old 12-04-2020, 10:01 PM
1969GiPper 1969GiPper is offline
Senior Chief
 
Join Date: Apr 2006
Location: Saint Clair Shores, MI
Posts: 275
Default

Primary and secondary rear brake shoes on backwards (reversed)?

  #4  
Old 12-05-2020, 12:40 AM
scott70's Avatar
scott70 scott70 is offline
Ultimate Warrior
 
Join Date: May 2002
Location: maine
Posts: 2,220
Default

Rear shoes are on correctly and no leaks. Yes the two valves I replaced are aftermarket. Don't believe I could find new oem valves. The brakes used to work fine. Then after sitting for a winter it started to lock the brakes if you hit the brakes at least semi hard at speed. Figured one of those proportion valves was sticking so I just replaced both. After that you could hit the brakes at speed and everything was good,,acts like it should. It only locks up now if I putt down the driveway and try to stop,,,it will drag the rear tires in the dirt. Maybe one of those aftermarket valves isn't working correctly? One of those valves is a hold off valve i believe.

__________________
72 lemans,455 e-head, UD 255/263 solid flat,3.73 gears,,,10" 4400 converter,, 6.68 at 101.8 mph,,1.44 60 ft.2007
(cam 271/278 roller)9"CC.4.11gear 6.41 at 106.32 mph 1.42 60 ft.(2009) SOLD,SOLD
1970 GTO 455 4 speed #matching,, 3.31 posi.Stock manifolds. # 64 heads.A factory mint tuquoise ,69' judge stripe car. 8.64 @ 87.3 mph on slippery street tires.Bad 2.25 60ft.Owned since 86'
  #5  
Old 12-05-2020, 01:08 AM
Scarebird's Avatar
Scarebird Scarebird is offline
Ultimate Warrior
 
Join Date: Nov 2002
Location: ABQ, USA
Posts: 4,998
Default

We usually recommend a simple adjustable valve; no hold off. Dial it in and forget it.

  #6  
Old 12-05-2020, 07:20 AM
steve25's Avatar
steve25 steve25 is offline
Ultimate Warrior
 
Join Date: May 2008
Location: Westchester NY
Posts: 14,633
Default

I have to assume that since you did not post the needed details that all was fine until the front brakes where worked on, no?

If so are you sure all the air was out of the front system?

It's the adjustment of the rear brakes that sets up the pedal feel and many times with these age cars the rear backing plates never got the proper grease job done during a brake job and as a result the shoe slid pads on the backing plate are worn and grooved.

When this takes place and after a brake job when you first hit the brakes the shoes don't / can't retract all the way, with the end result being that you just breath on the pedal and the brakes are already near locking up!

__________________
Wernher Von Braun warned before his retirement from NASA back in 1972, that the next world war would be against the ETs!
And he was not talking about 1/8 or 1/4 mile ETs!

1) 1940s 100% silver 4 cup tea server set.

Two dry rotted 14 x 10 Micky Thompson slicks.

1) un-mailed in gift coupon from a 1972 box of corn flakes.
Two pairs of brown leather flip flops, never seen more then 2 mph.

Education is what your left with once you forget things!
  #7  
Old 12-05-2020, 03:05 PM
1968GTO421's Avatar
1968GTO421 1968GTO421 is offline
Ultimate Warrior
 
Join Date: May 2014
Location: Travelers Rest, SC
Posts: 1,285
Default

Quote:
Originally Posted by steve25 View Post
I have to assume that since you did not post the needed details that all was fine until the front brakes where worked on, no?

If so are you sure all the air was out of the front system?

It's the adjustment of the rear brakes that sets up the pedal feel and many times with these age cars the rear backing plates never got the proper grease job done during a brake job and as a result the shoe slid pads on the backing plate are worn and grooved.

When this takes place and after a brake job when you first hit the brakes the shoes don't / can't retract all the way, with the end result being that you just breath on the pedal and the brakes are already near locking up!


Thankfully places like our sponsor, Ames, carries new backing plates (Left and Right side ones) to solve this problem.

https://secure.amesperf.com/qilan/De...A&web_access=Y

Thanks Steve for pointing out this little noticed problem.

__________________


"No replacement for displacement!"

GTOAA--https://www.gtoaa.org/
  #8  
Old 12-05-2020, 03:52 PM
Scott Thelander's Avatar
Scott Thelander Scott Thelander is offline
Ultimate Warrior
 
Join Date: Sep 2006
Location: Lacey WA
Posts: 1,558
Default

this is also a HUGE issue on the fronts also
I have had a few backing plates welded up refinished and reused them with no issues

  #9  
Old 12-08-2020, 07:11 AM
steve25's Avatar
steve25 steve25 is offline
Ultimate Warrior
 
Join Date: May 2008
Location: Westchester NY
Posts: 14,633
Default

That's certainly a good option ( welding ) and the weld is many times harder then the steel of the backing plate so even without lube the pads on the Brake shoes will ware far faster!

__________________
Wernher Von Braun warned before his retirement from NASA back in 1972, that the next world war would be against the ETs!
And he was not talking about 1/8 or 1/4 mile ETs!

1) 1940s 100% silver 4 cup tea server set.

Two dry rotted 14 x 10 Micky Thompson slicks.

1) un-mailed in gift coupon from a 1972 box of corn flakes.
Two pairs of brown leather flip flops, never seen more then 2 mph.

Education is what your left with once you forget things!
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -4. The time now is 04:20 AM.

 

About Us

The PY Online Forums is the largest online gathering of Pontiac enthusiasts anywhere in the world. Founded in 1991, it was also the first online forum for people to gather and talk about their Pontiacs. Since then, it has become the mecca of Pontiac technical data and knowledge that no other place can surpass.

 




Copyright © 2017