A master cylinder can create something that looks like a pump up, but
it really isn't. All a bad master cylinder can do is leak fluid past the seal.
This results in a sinking pedal. Not a low firm pedal, but one that
creeps down. The seal will leak and the pedal will sink under mild
brake application. Most people will not notice that the pedal is
sinking, they will only realize the pedal is low some time after it has
begone sinking. They will then reapply the brake, but will do so with
much higher pedal pressure. The higher pressure pushes the seal lip
harder against the bore, sealing it. and giving a non-sinking firm pedal,
but only when the brake pedal is pushed harder then normal.
There's a very nice article on master cylinder diagnostics at this web page
http://www.babcox.com/oop/qfullhit.htw?CiWebHitsFile=%2Fbf%2Fbf109954%2Ehtm&CiRestriction=master+cylinder&CiBeginHilite=%3Cstrong+class%3DHit%3E&CiEndHilite=%3C%2Fstrong%3E&CiUserParam3=/search.asp&CiHiliteType=Full
>>> "Bowen, Patrick" <pbowen@intellinetics.com> 10/12 11:13 AM >>>
Nolan, Bad Master Cylinders will most definitely pump up. While the
suggestions you gave are valid, most of the problems I have ever seen like
this are the M/C.
Patrick Bowen
-----Original Message-----
From: Nolan Penney [mailto:npenney@mde.state.md.us]
Sent: Thursday, October 12, 2000 9:21 AM
To: spitfires@autox.team.net; Gosling_Richard_B@perkins.com
Subject: Re: What's wrong with my brakes?
Being able to pump up the brakes is not normally an indicator of a defective
master cylinder, it's more commonly an indicator of maladjusted brake shoes.
It can also indicate a problem with the calipers.
In any case, you're first push on the brake pedal pushes the brake
components
out to the drum (or against the rotor). The second pump happens faster then
the brake components retract. Hence the pump up. And, hence the
repeatablilty when you give the brake components a chance to fully retract.
A bad master cylinder will not pump up.
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