[DMCForum] Re: Urgent brake problem -- Bench Bleeding Before Installatio
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[DMCForum] Re: Urgent brake problem -- Bench Bleeding Before Installation

This is why people "bench bleed" a master cylinder before installing
on the car and bleeding the rest of the system as usual. You don't
need to install the thing with reserviors FULL of fluid, but there
must be enough above the cylinders to compensate for leakage while the
brake lines are attached. Makes a tremendous mess (that needs to be
cleaned up before you can ensure fittings aren't leaking), but what
else is a man to do?

Most master cylinders are terribly prone to rust pitting, BTW.

Bill Robertson

>--- In DMCForum@xxxxxxxxxxxxxxx, Martin Gutkowski <webmaster@xxxx> wrote:
> I was under the impression that you cannot really rebuild brake master 
> cylinders. Here's why: When you use the clutch, you depress the master 
> cylinder throughout the whole length of its travel every time you use 
> it. This keeps the bore of the cylinder clean and wear is uniform 
> thoughout its length. We sell a clutch master cylinder rebuild kit.
> The brake master is a completely different matter. You use the brake 
> master through a fraction of its length throughout it's entire working 
> life. The only time you will use it more is when something's going
> or when bleeding the system. If you're not careful, you can bust a 
> perfectly good MC when bleeding the brakes simply by pressing the pedal 
> to the floor, because the piston moves into the area of the cylinder 
> that never gets used, and a poorly maintained sustem may well have 
> corrosion and general crap built up on the walls. One movement too far 
> and bingo splingo, busted seals. A "rebuild" would involve cleaning and 
> replacing seals, but can't repair damage to the cylinder walls. For
> reason we sell only new brake master cylinders.
> > With about 3 more pumps, it firms up
> >properly and stays firm for as long as we continue to pump
> >it.  But if we let it sit for more than 15 seconds then the
> >pedal goes soft again.  
> >
> You have got the engine on? This behaviour is exactly what you'd expect 
> from the servo when the engine's off.
> Martin

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