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 Brakes locked up, then failed...ideas?

 Created by: OKMini
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 Posted: Aug 20, 2018 08:37AM
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CA
Does the car have the brake switch mounted on the pedal box? I had a car where the brake light switch was screwed in too far, not allowing the pedal to return fully, which would not allow the fluid to return, and ultimately locking the brakes.

Something to check, it took me ages to figure that one out

Sean Windrum

1996 MGF VVC
1970 1275 GT Racer
66 Austin Countryman
63 997 Cooper (Under Construction)
63 MG 1100

 

 Posted: Aug 20, 2018 04:36AM
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"Stevejohn" IS the scammer of the day. Don't follow his link.

.

"Hang on a minute lads....I've got a great idea."

 Posted: Aug 19, 2018 06:39PM
 Edited:  Aug 19, 2018 06:47PM
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Regarding Kelly's post; per Seven Enterprises, the length of the pushrod for a GMC159, used in a mini, is 3.5" tip to tip. The longer pushrod was part of a kit offered by BMC Special Tuning to convert to tandem front/rear braking.  For what its worth, ST also specified that BOTH front and rear brakes be bled AT THE SAME TIME, (Their emphasis, not mine) requiring "three operators and two sets of bleeder equipment."  I bled my 159 equipped car using the normal method with no problems.

 Posted: Aug 19, 2018 05:31PM
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Picture attached.  Thanks for the responses.

The spacer is in place below the MC, and I have no reason to believe this MC is not original to the car.  I appreciate that failing to put all the pieces back in, but I don't think this is the case.  It all worked well for the various short trips to this point.  I did notice that the fluid seems discolored so maybe it has picked up from the old lines I reused and got some crap in the MC.  Appreciate any suggestions on anything to check out.

1976 1000 (Current Project)

1975 1000 Donor

1969 Cooper (Future Project)

1971 Opel GT

1972 Corvette Stingray

 Posted: Aug 19, 2018 02:25PM
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I have seen this happen when a GM159 type master cylinder from an 1100/1300 ADO 16 was used on a Mini. The master cylinder push rod is longer on the ADO16 version and kept the brake fluid from being able to reliably return to the reservoir. I've been told you can either fit the shorter push rod or shim master cylinder up so it sits a bit higher to allow the master cylinder piston to fully return.

Kelley

"If you can afford the car, you can afford the manual..."

 Posted: Aug 19, 2018 02:12PM
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If you just rebuilt the Mini, is there a chance you left something out that you shouldn't have? Here's something to think about. There should be a piece of metal that sits under the Master Cylinders on top of the bulkhead. If you leave that piece of metal out, your brakes will lock up after only a few hits. Some folks think the piece of metal (spacer) is not really critical, but it is. So, could this be your issue? Not sure why you can pump them up and then loose pedal. Photos of the Master Cylinder area please?

"Retired:  No Job, No Money, Wife and I!  Will travel anywhere for Minis"

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 Posted: Aug 19, 2018 11:16AM
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If you can pump the brakes and they work with a firm pedal... and the pedal does not sink to the floor... it is likely to be a problem with the way the shoes were installed and adjusted, not the MC. 

I cannot tell you what to look for with PDWA failures except it's easy to mess up their spool centering during brake bleeding.  Actual PDWA failures can be seen with things like fluid coming out of the switch.

Doug L.
 Posted: Aug 19, 2018 09:21AM
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Took the newly finished Mini 1000 on its first lengthy drive to a show 1/2 hour down the highway.  Drove like a champ, but on the way home, the front brakes locked up and by the time we stopped, both sides were smoking.

When the brakes released, the pedal goes straight to the floor, no brake front or back.  The pedal can be pumped to bring pressure back up and the brakes will work, and will hold until the pedal is let back up.  Then instantly, will go back to the floor and no brakes at all.

There are no signs of leaks anywhere.  with the exception of the lines transverse metal lines at the back and front, almost everything is new.  Master cylinder was rebuilt with the proper kit and the PDWA valve and inertia valve were disassembled and seemed to be ok.  It's a front/rear split system with a GM159 MC.

I suspect this is a MC problem, maybe an internal leak.  I will start with a system flush and re-bleed everything to see if some air worked it's way back to the MC, but also wanted to see if anyone has any ideas on any other possible cause or things to check.

Thanks,

Corey

1976 1000 (Current Project)

1975 1000 Donor

1969 Cooper (Future Project)

1971 Opel GT

1972 Corvette Stingray

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