• Hello Guest, welcome to the initial stages of our new platform!
    You can find some additional information about where we are in the process of migrating the board and setting up our new software here

    Thank you for being a part of our community!

Failed Booster Locks F.Brakes ?

B

bennyg

Member
Joined
Oct 22, 2010
Location
Hi plains Big WindyWHY-Ohh
I had a stuck caliper on my 1991 740 w/ABS .I have never seen a piston so locked up ! I changed the caliper , but lo n behold , it was still locked up while the engine was running making vacuum.I turned the engine off and came back ,and the front brakes loosened . Next thing I did was remove the brake booster check valve to the booster. Upon removal the valve broke and a small piece of it fell inside the booster ! With the vacuum supply removed and plugged to the booster ,the brakes do release ,as you would suspect the brakes do not operate as they should , but it will keep the car moving until I figure out the final cause of the problem. .I have a few questions ,as I am basically CLUELESS as to the mystery that happens inside a brake booster . 1.] Has anyone out there in Bricklyn ,Brickdale ,or Brickville ever experienced this phenomenon of a failed booster locking up just the front brakes ? I dont think the rears were locked up , however they may have been , but due to the fact that the differential was turning/overcoming the resistance . It is not a fun drive on slick winter roads ...Does anyone understand the booster function and failure symptoms .2.] Will the small piece I dropped inside the booster housing cause me grief ?
 
I turned the engine off and came back ,and the front brakes loosened .
Click to expand...
Sounds like something got inside the booster and is keeping the internal valves from sealing properly, or something is pushing on the pedal (carpet? wire harness? missing basketball?).

Does anyone understand the booster function and failure symptoms
Click to expand...

Copied from my post a year ago:

http://forums.turbobricks.com/showthread.php?p=5040322#post5040322

Internally, it has 2 chambers (front and rear), separated by a diaphragm that's attached to a large moving plate that connects directly to the pushrod/master at the front, and the brake pedal at the rear (via an internal valve that's part of the plate).

At rest, the valve is open between the chambers so they both have the same amount of vacuum in them, and outside air is prevented from entering. When you press the pedal, the valve's movement first closes the port between the front and rear chambers, then opens another port that allows air into the rear chamber (through the snout at the rear, where the pedal attaches). The difference in pressure causes the plate to move forward, pushing the pushrod into the master, and applying the brakes.

If any part of the valve sticks open or closed when it shouldn't, you'll get odd behavior from the system; either no assist, too much assist, hissing from constant flow, or even brakes that "apply themselves". Moisture build-up inside the booster can also affect things, if it gets cold enough to freeze. Even a drop can hold a valve open, while larger amounts in a chamber can interfere with the plate's movement. If there's a foam air filter in the rear snout (there should be, inside the rubber bellows), it could be falling apart, letting chunks of itself get sucked into the booster.

If a leak is small enough that it's "on the threshold", high vacuum from deceleration or idle could be enough to apply the brakes, while stepping on the gas (reduces intake manifold vacuum), could be just enough to temporarily release them.

Here's a very simplified "animated" booster so you can see how it all works.
Half-way down, play with the buttons....

http://www.kfz-tech.de/Engl/Bremskraftverst.htm

Will the small piece I dropped inside the booster housing cause me grief ?
Click to expand...
If it's sharp enough, it could cut a hole in the rubber diaphragm and cause an internal air leak. Or if it's big enough it could jam the big piston and keep you from applying the brakes. Not good either way.
 
My 760 was doing that. Not locking entirely, but near. And I think it was rears. Can't remember, it was years ago.
Unpluging booster made it ghetto drivable until I swaped a good one in, which restored full functionality of the brakes.
 
The booster will have no effect on front vs. rear locking, just whether or not there is still pressure on the master cylinder or not.

I'd guess a failed booster at this point or a very slim chance the pushrod extended somehow. That of course is assuming you don't have an issue with the pedal being held partially down.
 
This all seemed to happen when I was retrofitting cruise control brake pedal switch , I checked to see if it is pushing on the pedal in the rest position , it seemed fine , and I also assumed if something was pushing it would activate brake lights , but it didnt .The back brakes may very well be locked , but were able to be overcome due to engine power , the front right however was locked up tighter n a Cunns Nunnt... I really think its frozen condensation etc inside booster as it has been colder N that proverbial witches mammary gland up here .As much as I am bummed out about my broken Volvo , Im so thankful that I can usually repair it on my own ,and not be at the mercy of a steelership etc ...Thanks for any and all input ...onward thru the fog n snow we go
 
gsellstr said:
The booster will have no effect on front vs. rear locking, just whether or not there is still pressure on the master cylinder or not.

I'd guess a failed booster at this point or a very slim chance the pushrod extended somehow. That of course is assuming you don't have an issue with the pedal being held partially down.
Click to expand...

Not a direct effect, anyway, but failed booster would heave my pedal a hair lower tha it should be, so I constantly had to pull it up with my leg. Failed booster -> pressure on the master -> constantly applied brakes. My brakes weren't fully locking, but were constantly dragging and heating up. I was rarwly driving that car at the time, so it took me a couple of drives to figure why was it acting like a dead dog...
 
bennyg said:
I had a stuck caliper on my 1991 740 w/ABS .I have never seen a piston so locked up ! I changed the caliper , but lo n behold , it was still locked up while the engine was running making vacuum.I turned the engine off and came back ,and the front brakes loosened . Next thing I did was remove the brake booster check valve to the booster. Upon removal the valve broke and a small piece of it fell inside the booster ! With the vacuum supply removed and plugged to the booster ,the brakes do release ,as you would suspect the brakes do not operate as they should , but it will keep the car moving until I figure out the final cause of the problem. .I have a few questions ,as I am basically CLUELESS as to the mystery that happens inside a brake booster . 1.] Has anyone out there in Bricklyn ,Brickdale ,or Brickville ever experienced this phenomenon of a failed booster locking up just the front brakes ? I dont think the rears were locked up , however they may have been , but due to the fact that the differential was turning/overcoming the resistance . It is not a fun drive on slick winter roads ...Does anyone understand the booster function and failure symptoms .2.] Will the small piece I dropped inside the booster housing cause me grief ?
Click to expand...

Did you diagnose for collapsed brake hose at stuck caliper?
 
I've seen 2 cases of the abs module under the hood being bad. It would lock up one rear wheel until the pressure bled off. Changed caliper, hose, master, etc... But it was the ABS.
 
I thought the abs didnt kick in until 12 mph ? The reason I am suspecting booster is due to the fact that they released when the engine was turned off and vacuum bled, they released...however now that I think about it when the old caliper was on and the engine off the right front was still locked ...So KZOC what did you do replace the ABS controller ? Im not a big ABS fan and could easily live w/o it
 
however now that I think about it when the old caliper was on and the engine off the right front was still locked
Click to expand...
It's possible you had multiple problems. A stuck caliper and/or a bad hose and/or a bad booster. Do you still have both old calipers? Do both pistons move?

Oh I almost forgot to ask ...What the heck is a missing Basketball ?
Click to expand...
It's that thing you played with last summer, that now you can't find, that somehow might have bounced under the dash and got stuck pressing against the brake pedal.

basketball005.gif
 
HHa Ha , good to see we can appreciate humor here ...but up here in the Tundra were too busy for all that Lollygaggin ...we have too much snow to shovel , n logs to haul etc etc , n no time for gladiators games etc.And furthermore ,I'm well aware ,as in life multiple disorders are possible . I probably made a cardinal sin of braking and just replaced one side , the r. side .After removal the r. side moved readily w/ air applied ... when no vacuum is applied brakes no lockie ...Makes me hate myself for crushing my parts car ...what was I NOT thinking ...sheeesh !
 
Oh and the caliper end of the brake hose and reducer fitting were froze up tight , it was all I could do to remount the old caliper w/o destroying hose n fitting further . Im going to replace the hoses asap , but dreading complete removal, due to galling/seizing of fittings ...
 
You must log in or register to reply here.
Back
Top