Brake bleeder pressures

[four.rings@MCIONE.com]

Eric wrote:

Does anyone know 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?
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

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.

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.

-- 
Igor Kessel
'89 200TQ -- 18psi (TAP)
'98 A4TQ -- nothing to declare
Philadelphia, PA
USA