Date:         Sat, 27 Mar 2010 13:16:41 -0700
Reply-To:     Don Hanson <dhanson928@GMAIL.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Don Hanson <dhanson928@GMAIL.COM>
Subject:      Re: Brake Proportioning Valve: Traps Air?
Comments: To: neil n <musomuso@gmail.com>
In-Reply-To:  <c4e7c5f91003271204p535fdbe2qbd2771559e445362@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1

  Dunno about a bleeder screw on the proportioning valve.but the
Proportioning valve is probably why your rear brakes bleed fluid more slowly
than the fronts.  The P valve is essentially a gate-valve that controls the
flow of your hydraulic fluid..restricting some flow towards the rear,
increasing the relative initial "action" of the front brakes.  Eventually
you will have even pressure at all the wheel cylinders, but that P valve's
function is to regulate or adjust how quickly the pressure can equalize...

  If you had no proportioning valve...as you stepped on the brakes you would
likely get rear wheel lockup.  You would have a 50:50 application of braking
force, front to rear, with no valve,   but when you apply the brakes the
vehicle weight shifts onto the front wheels.  So, the P-valve starts the
braking at the front wheels and then, depending on the valve, lets  brake
force take hold at the rear wheels more slowly.

  Don Hanson



On Sat, Mar 27, 2010 at 12:04 PM, neil n <musomuso@gmail.com> wrote:

> Hi all.
>
> Searched but didn't find an answer.
>
> My '81 Vanagon proportioning valve:  http://tinyurl.com/y8q7e3x  is
> similar or the same as those used on later T2's. My T2 Haynes says
> that some MY's (pre '73?) have a bleeder screw. (there's a nice
> drawing on P 130)
>
> I understand the valve design changed within the T2's, and again in
> the T3's, but reading of this bleeder screw made me curious.
>
> Are the older T3 proportioning valves prone to trapping air bubbles
> when hydraulic system opened/serviced? If so, is there any use in
> bleeding valve at the connection(s)?
>
> Likely not I bet. And possibly risky since nut and line might be
> "welded" to valve with rust. Something a shade tree might want to
> leave alone til' line replacement required.
>
> Neil.
>
> --
> Neil Nicholson '81 VanaJetta 2.0 "Jaco"
>
> http://tubaneil.googlepages.com/
>
>
> http://groups.google.com/group/vanagons-with-vw-inline-4-cylinder-gas-engines
>