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Re: Brake bleeder pressures, etc.

>I think there has been some misundertanding of Eric's original question:  
>     "... what kind of fluid pressures the brake MC generates when you
>      stand on the brakes (like in a 60-0mph test)?  I believe it would
>      be easily in excess of 20psi, no?"
>However, I think he was really interested in knowing what kind of pressures the
>seals in the MC _reservoir_ see, in the context of pressure bleeding the brake
>system.  Yes, the pressures in the brake hydraulic system could easily be of
>over 1000+ psi, but the seals at the brake reservoir cap don't normally see any
>pressure at all. ... So yes, when you pressurize the reservoir to 20 psi you
>are actually exceeding the design values by an order of magnitude.  It won't
>take much pressure to pop the reservoir off the MC.  

I was apparently thinking of the wrong seals, then, as I was asking about
the seals in the MC itself.  Perhaps the Coupe resevoir is different
than most, because I highly doubt that you can pop the resevoir off of the
MC (and out of its seals) with a measly 20psi.  It takes two strong arms
to separate the two when they are sitting in my lap.

>Again, the seals that give are not the MC piston seals, but the seals on top of
>the MC, which join the reservoir it to (and most likely are included with a new
>reservoir or MC).  The MC piston seals won't even know that the cap has been
>presurized to 10 psi since it was designed to  tolerate >1000 psi on a regular
>basis.  On loose fitting reservoirs, even 10 psi may be too much.  Remember you
>are exeeding the design pressures by an order of magnitude!!  Better to strap a
>tie-wrap around it just to be safe! Even if you are using "only 10 psi", the
>brake fluid pressure could be pushing the reservoir up with a force of 2 lbs or

Ok, I can see where some people may be concerned about their resevoir
seals.  I would not expect them to come off, however you may see them
leak under pressure if they are too old.  The 13 y/o resevoir seals in
my Coupe GT are in pretty decent condition, though, and hold 20psi with
no problem.

>The same applies to the clutch hose.  Although rare, if the brake fluid
>reservoir is overpressurized, the reservoir itself may stay in place, but the
>clutch hose could pop at either the reservoir or the clutch MC end and the
>engine compartment will be filled with brake fluid in a hurry.  Not fun.  

Yep, BTDT.  The hose popped off the resevoir and brake fluid spilled
everywhere (I was using a different method at the time and wasn't
paying as much attention as I should).  No resevoir seal leaks, even
though I am sure they saw >25psi pressure.  That isn't to say that everyone
can do this, so anyone doing it should be careful.

>I don't want to discourage people from pressure bleeding their cars, since it is
>extremely easy and convenient, but they need to be aware of the risks before
>trying it.  Just strap the reservoir in place and maybe  put a small clamp
>around the clutch hose and keep the pressures low.  Don't even think about
>hooking up a compressor at 100psi directly to it.

Well said.  If you don't feel safe @ 20psi, then try 5 or 10psi.  If you
think your seals are in good condition, then 20psi may not be too much.
I don't think you have to worry about blowing the resevoir off of the
MC, though, with 20psi or less pressure.

'85 Coupe GT
Eric J. Fluhr                                Email:  ejfluhr@austin.ibm.com
630FP Logic/Circuit Design                   Phone:  (512) 838-7589
IBM Server Group                             Austin, TX