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Re: Brake bleeder pressures, etc.

Igor Kessel <four.rings@MCIONE.com> wrote:
> You are right, Eric, it is in excess of one tonne per side. However,
> there's no contradiction.
> The MC seals have to withstand just the foot's pressure. It's the
> calliper that develops this tremendous force. The car's brake (and
> clutch) hydraulic sys is a regular hydraulic press:
> you lose in travel but win in the force. The modest foot force applied
> to the 22mm dia piston in MC displaces the (approx.) 100mm long
> "cylinder" of brake fluid. On the other end two (approx. 50mm dia,
> guesstimating from memory here) calliper pistons have to travel barely
> 0.5mm, developing a tremendous force in the process. Easily
> calculable, if it were not already 23:30 on my computer clock.

The pressure in a fulid is the pressure in a fluid, give or take some small
gravity contributions.  The pressure that the MC seals see is the same 1500+ psi
that the front calipers seals see (the pressure in the rear callipers is
regulated down).  The _force_ in the MC piston is much smaller than the force in
the caliper pistons, due to the fact that it is a product of fluid pressure and
piston area.  The MC piston is much smaller, so you only need to apply
relatively little force to the pedal to have huge forces applied to the brake

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.  Maybe a very slight vacuum as the fluid goes down as the pads
wear.  The seals at the bottom of the reservoir don't normally see any pressure
either - only a little head pressure generated by maybe one inch or two of brake
fluid - maybe 0.5 psi??  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.  

> Now, if you apply too much pressure to the MC piston, which seals were
> never designed to deal with such level of force, that entirely
> different thing. Although I've never seen the seal blow-out, I've been
> warned about it by several professional garage mechanics. I'd rather
> keep it at the modest 10psi level and spend 30 min bleeding the
> brakes, than save 15min by increasing the pressure twofold at the risk
> of expensive blow-out. Folks, keep in mind that our MCs are not
> repairable, the seal kit is not available and the whole MC assy is
> permanently sealed with epoxy, unlike some other MC, say, that of my
> 320i which I had successfully repaired by replacing the seal kit in
> 1989. Now, that's one feature that I like in the Bimmers better. 
> I did sell the Bimmer within a few weeks of ownership, tho. I found it
> to be rather small, crude and slow. Instead  I bought a FWD 5-sp
> 5000s, which was big, elegant and fast. Never regreted it.

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

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.  

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.


Luis Marques
'87 4kcsq