[Rovernet] Brakes locked 3500S only when car is on - Booster?

[Steve Manwell]

Rob and Steven,

There is also a small (1/4-inch?) piston between the hydraulic side of 
the master cylinder and the air valve/diaphragm assembly.  I haven't 
studied the details of the master cylinder for while, but I think you 
might get the same result if this piston does not return after the 
brakes are released.

This small bore is often neglected when the larger bore is sleeved for a 
rebuild.  I also found out that some rebuilders will sleeve the outside 
of the large piston to get a good seal.

Steve


On 7/23/2014 1:25 PM, Steven Dibdin via Rovernet wrote:
> Rob, I would check that the actuator for the diaphragm in the white plastic cap is moving freely. If it's stuck in the up position it'll give you just the problems you described.
>
> Hope that helps,
>
> Steven
>
> Sent from a small screen, please excuse any typing errors.
>
>> On Jul 23, 2014, at 11:37, Rob Sayre via Rovernet <rovernet at rovernet.org> wrote:
>>
>> Good morning:
>>
>> When I start my 1970 3500s the brakes lock and the pedal is hard as if applied.
>>
>> If the vacuum line from the intake to the booster is disconnected everything frees up.  All lines from the intake and between the booster and the master cylinder with the white plastic valve assembly are intact with no leaks.  The little valve under the cap with the spring is not leaking any air and is pulled closed by vacuum.
>>
>> I took the white plastic assembly apart and the diaphragm  has no visible defect.
>>
>> The car sat for several years.
>>
>> Any help on how to pinpoint and fix the problem would be appreciated.
>>
>> Thanks,
>>
>> rob
>>
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