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jonathon wrote:
> I thought the reason for the residual check valve was to keep a small
> pressure on the cups in the wheel cylinder to keep it from leaking. Some
> cups had spring backings that eliminated the need for the residual pressure.
> It's not just a check valve, it maintains a small (residual) pressure on
> it's output, until it leaks down of course.
That's correct, Jonathan. A "standard" 10 psi RPV for drum brakes will NOT prevent
brake shoes from retracting. A typical brake shoe return spring is strong enough
that it could require anywhere from 60 to 90 psi of line pressure to start
expanding the shoes outward. The shoes are kept close to the drum by ADJUSTERS of
various configurations, NOT by the RPV.
I've seen a couple of brake parts catalogs that had this wrong, but most seem to
have it correct. There are plenty of other Internet and print sources that will
confirm this. In addition, I've checked the line pressure on my M37 brakes to see
what it takes to move the shoes, and depending on the condition of the return
springs, it varied between 65 and 85 psi, which is far above what a typical
residual pressure valve would maintain in the system. The shoes retract as if the
RPV wasn't even there! And this seems to have always been the case on the 40-odd
vehicles that I've owned and worked on...so far at least. IF there IS some system
out there that uses high-pressure RPVs that can keep the shoes out against the
drums, I'd like to know exactly what system it happens to be so that I could learn
more about it.
By keeping the wheel cylinder cups snug against the sides of the bore, it not only
helps prevent seepage, but it also helps prevent AIR from being drawn into the
system around the wheel cylinder cups. When you slip your foot off of the brake
pedal and the pedal pops up, the return springs in the master cylinder try to push
back the master cylinder piston faster than the line pressure from the returning
brake shoes can push it back. This can create a brief negative pressure pulse that
could allow the wheel cylinder cups to relax, allowing air to be drawn in around
the cups...unless they have a bit of positive pressure on them.
A replacement or adjunct approach to the RPV is an "expander"...a small metal part
that presses outward on the wheel cylinder cups to keep them tight against the
cylinder bore.
As to a master cylinder, the MC is designed to handle negative pressure pulses by
allowing fluid to pass from a second, fluid-filled chamber behind the
high-pressure seal through little holes in the master cylinder piston and around
the high-pressure seal. The negative pressure in this second chamber is quickly
relieved by the relatively large "breather hole" between the chamber and the
reservoir. A dual master cylinder typically has two of these setups. If the
pressure in this chamber drops enough to draw a bit of air past the rear-most seal
into the chamber, it generally will quickly and purge itself through the breather
hole into the reservoir.
Here are a few references, in case anyone is interested:
Brake Handbook, by Fred Puhn, published by HPBooks, 1985, New York
Motor Truck Repair Manual, by Motor Publications, various years, New York
A few Internet sites, including:
http://www.hotrodsworldwide.com/vendors/eci.htm
http://www.hotrodssuperstore.com/super-store/whattypevalve.html
http://www.autosite.com/GARAGE/encyclop/ency14b.asp
Alan Bowes
(The comments above only represent my personal opinion, with no guarantee of
accuracy.)
>
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