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1976 all brakes slowly bind after driving


Texer

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Hi - first post here, looking for advice on a brake problem on my newly acquired ‘76 2002. Hoping this is an ok spot to post a question.

 

Issue:

Front brakes get progressively tighter as I drive.

 

If the car has been unused for overnight, the front wheels spin freely and normally (I have tested this by raising the car and spinning both the left and right front wheels by hand easily with an index finger)

 

If the car has been unused for hours the brake pedal has what feels to me like normal travel.

 

After driving in town or on highway for about 10 miles, the brake pedal stiffens. 

 

After driving in town or on highway for about 10 miles , the front brakes also begin to apply by themselves.  I notice the resistance while driving where the car will brake itself with no engine load in neutral.  It comes to a stop by itself and dips at the end of travel.  It can also hold itself from rolling on a slight incline/decline while in neutral with no pedal pressure while in this state. 

I have verified the situation after stopping the car in this circumstance and finding that the front wheels are binding heavily (I have tested this by raising the car immediately after the test drive and have been completely unable to spin both the left and right front wheels.  This is when grasping the tires and forcing with all my effort = no rotation movement.

 

i by retested the situation and found the rear brakes are also affected in the same way. 
 

The car has been to a specialist and the issue was not repeatable in the visit. It was then checked by Les Schwab, they said the brakes are great, but did not drive more than a block on testing. 
 

The car has a Weber system; would it be possible that the vacuum is creating this issue? Pics of carb hoses for reference. 

 

thanks in advance for any advice. 

 

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90A1F0EC-8B93-4439-BFA8-7B3ED10E9F7F.jpeg

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My first thought is the mechanical linkage between the brake pedal and booster is hanging up, try squirting a little light oil on the moving joints and see if it loosens up, other things that can bind the linkage are worn bushings in the pedal box and carpet interfering with the pedal.

If everybody in the room is thinking the same thing, then someone is not thinking.

 

George S Patton 

Planning the Normandy Break out 1944

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I had a brake booster fail internally and cause the brakes to self apply with engine vacuum, but the car was essentially un-drivable when it happened and I had to get a tow. In hindsight, I probably could’ve disconnected the vacuum line and nursed it home, but didn’t want to be troubleshooting on the shoulder of the freeway.

 

Not sure whether that could happen in a less extreme state to just cause dragging rather than the near lockup I experienced. 
 

Brent

1974 2002 - Megasquirt and turbo

2018 BMW M2/ 2013 Porsche Cayenne Diesel

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When it does it try reaching down (while stopped) and see if you can pull the brake pedal up any. Hans is on the right track if one brake is hanging up it's most likely that brake more than one doing the same thing it's almost always the M/C.

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If everybody in the room is thinking the same thing, then someone is not thinking.

 

George S Patton 

Planning the Normandy Break out 1944

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I'm going with the booster is not allowing the MC to return.  I discovered this when I replaced everything for the pedal to the pads in the brake system.  The only thing I didn't replace, at the time, was the brake booster.   I ended up replacing that also.  Problem was solved.   

 

fwiw,

Matt

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1 hour ago, Schnellvintage said:

I'm going with the booster is not allowing the MC to return.  I discovered this when I replaced everything for the pedal to the pads in the brake system.  The only thing I didn't replace, at the time, was the brake booster.   I ended up replacing that also.  Problem was solved.   

 

fwiw,

Matt

Could it have something to do with that hose?

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59 minutes ago, Hans said:

Could it have something to do with that hose?

I'm guessing that 'lump' in the hose is a type of one-way valve for the brake vacuum?  I know mounting it bass-ackwards has the tell-tale sign of white smoke in the exhaust on deceleration. If it's clogged, you'd expect no boost from the brakes, not increasing boost, no? Unless his booster is not releasing pressure somehow.

 

Given it's a '76 - could it be something odd/failing with the 76-only brake pressure valve? It's supposed to detect the pressure difference between the circuits - could it fail in a way to prevent flow back to the MC?

Where we goin’? … I’ll drive…
There are some who call me... Tom too         v i s i o n a u t i k s.com   

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never heard it happen on BMW before, but on other cars this symptom is always the brake booster, normally the air valve doesnt close properly so the brakes are partially on all the time. If you disconnect & block the vacuum pipe, then that isolates the booster... car should still drive...beware brakes will require a lot more foot pressure...but the binding should go away. That proves its the booster causing the brake drag, and it needs an urgent repair or replacement.

'59 Morris Minor, '67 Triumph TR4A, '68 Silver Shadow, '72 2002tii, '73 Jaguar E-Type,

'73 2002tii w/Alpina mods , '74 2002turbo, '85 Alfa Spider, '03 Lotus Elise

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34 minutes ago, visionaut said:

Given it's a '76 - could it be something odd/failing with the 76-only brake pressure valve? It's supposed to detect the pressure difference between the circuits - could it fail in a way to prevent flow back to the MC?

I don't think so the valve has a piston for each circuit but they don't drop into the flow of fluid in anyway I can recall. Although I haven't looked at it considring that it could.

If everybody in the room is thinking the same thing, then someone is not thinking.

 

George S Patton 

Planning the Normandy Break out 1944

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1 hour ago, visionaut said:

I'm guessing that 'lump' in the hose is a type of one-way valve for the brake vacuum?  I know mounting it bass-ackwards has the tell-tale sign of white smoke in the exhaust on deceleration. If it's clogged, you'd expect no boost from the brakes, not increasing boost, no? Unless his booster is not releasing pressure somehow.

 

Given it's a '76 - could it be something odd/failing with the 76-only brake pressure valve? It's supposed to detect the pressure difference between the circuits - could it fail in a way to prevent flow back to the MC?

I was thinkin g of its construction - is it proper vacuum hose? Not sure if that could be a factor.

I have this faint bell ringing about the booster not releasing.

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I had that exact thing happen on my Karmann Ghia years ago. The cause was internal deterioration of the flexible rubber sections of the brake lines,  As you apply the brakes it allows fluid to move forward but it would not allow it to return. Brakes got tighter and tighter until I ended up in a rest stop on the side of the highway with HOT wheels! 

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Thanks to all for your replies; I have no problem with brake hose replacement, etc, but I’ll move first with the linkage recommendations, and Booster check first. I did remove the cover-to-booster hose earlier and blew through it. Obviously one way was clear. The other direction was possible to push air through but it would be intermittent meaning it would lock up and then pass air with a new push of air. I’m thinking that may be the normal work of that valve, but have now previous experience to to draw on. Thanks again.

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