[s-cars] Brakes seize up when engine is warm

[Matthew Russell]

Master cylinders can and do fail to release hydraulic pressure. Usually due to a clogged return port or bad seals in the master, BTDT. 

This is what causes the brakes to gradually lock up. Compounding the problem is the heat generated at the caliper eventually heats up the brake fluid which then builds pressure which has no place to go. 

Vicious cycle. 

But I do agree with Tom. You need to determine which item is causing the problem, m cyl or booster.

-Matt, CO
S6a

Sent from an iDevice. Sorry. 


On Apr 13, 2012, at 7:44 PM, s-car-list-request at audifans.com wrote:

> Message: 3
> Date: Fri, 13 Apr 2012 15:57:53 -0400
> From: Tom Green <trgreen at comcast.net>
> To: "s-car-list at audifans.com list" <s-car-list at audifans.com>,
>    trodri69 at gmail.com, "Steven B. Mills" <s.b.mills at gmail.com>
> Subject: Re: [s-cars] Brakes seize up when engine is warm
> Message-ID: <335F6F6A-3C5E-4403-9FA4-74514194AEDA at comcast.net>
> Content-Type: text/plain;    charset=US-ASCII;    format=flowed;    delsp=yes
> 
> You have received several replies from knowledgeable folks on this  
> list, Tom,
> and I value their opinions on things s-car.  It doesn't fit with my  
> experiences,
> but they were not with the UrS6 or equivalent brake system.
> 
> The experience that most closely compares to your issue happened on a  
> Rover
> with vacuum boost and your description mirrors that experience  
> exactly.  The
> dealer replaced the booster they diagnosed as leaking (under warranty)  
> which
> solved the problem.
> 
> The master cylinder failed on my MG and resulted in no brakes unless  
> there was
> a frantic pumping of the pedal and use of the cable operated emergency  
> brake.  I
> see this as the classic master cylinder failure because this is a  
> failure of the seals
> which are the only working parts of the cylinder.  This vehicle did  
> not have boosted
> brakes, but I don't see where a failure in the master cylinder would  
> produce pressure
> to apply the brakes.  There is fluid expansion with heat, but it would  
> take a lot of heat
> to create the pressure required to lock up the brakes as you describe.
> 
> I have had a rusty emergency brake cable fail to release and keep  
> sufficient pressure
> on the brake pads to heat up the caliper sufficiently to lock up a  
> rear brake.  The front
> brakes will appear to be more sensitive when some of the pressure is  
> not required for
> the rear brakes, but not the way you describe your braking problem.  
> ( 5000tq)
> 
> Your description of the grabby (sensitive) nature of the brakes points  
> more to a leaking
> brake booster that just requires a little time (rather than heat) to  
> pressurize the master
> cylinder enough to start applying the brakes.
> 
> You have asked for advice, and I don't have much other than suggest  
> expert help.  The
> only diagnostic path for a DIY job would be to remove the hydraulic  
> boost pressure from
> the servo to see if the master cylinder applied brakes by itself.   
> There is not a practical
> method to do this and it is not safe to drive without brake assist.   
> So, you could change
> one or the other and see if that fixes the problem.  The master  
> cylinder is the obvious
> choice, both for cost and labor required, and the previous diagnosis  
> is probably correct.
> : )
> 
> You should also investigate where the hydraulic fluid escaped from the  
> system;  Even
> though not the cause of this issue, it will result in many other  
> problems.
> 
> Tom '95 S6
>          '95.5 S6 avant
> Knoxville, TN