Date:         Wed, 24 Mar 2010 17:31:42 -0700
Reply-To:     Scott Daniel - Turbovans <scottdaniel@TURBOVANS.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Scott Daniel - Turbovans <scottdaniel@TURBOVANS.COM>
Subject:      Re: Proper Brake Pedal Feel/Travel?
Comments: To: neil n <musomuso@GMAIL.COM>
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hi,
I have not heard of the rear backing plates flexing thing.
I have seen them rusted to 'almost nothing' though ...
but if yours are intact and look normal ..
I doubt you have a problem there.

what I am going on is ...
there is a deal where you can press down on a brake pedal, ( on a dual
master cylinder ) and you feel like it hits something hard inside the MC.

and this is how I think a dual chamber tandem MC works...
your foot pushes the first piston ....developing pressure,
now I don't know if the second piston is pushed by a rod or some connection
between the first piston and the second one ( I doubt it ) or if hydraulic
pressure from the first piston pushes on the second ( for the rear brakes )
piston. ...
but seems like it might be built to work that way.

so...if there is not a good seal within the MC between the two tandem
chambers ...
you get that feeling in the pedal..full hydraulic brake force is not being
achieved to the wheels...
and the pedal bottoms out on something hard .....I suspect that's the second
piston you are feeling, which you are not ever supposed to feel.

Looking on page 47.2 of the B. Bible ...
it shows an exploded view of the MC.
I notice there's a primary and secondary piston.
I see a 'stoke limiting screw' ...and also a 'stop sleeve.'

anyway ...contemplate that page and diagram.

oh here is something that would be nice to know...
but is rarely done...
that is to measure actual hydraulic pressure in psi at each wheels..or on
the front line and the rear line.  Rarely ever done though.
there are even scales that can measure how hard to pads are squeezing a
disc....but that's rarely done so hardly an option.

here's a cheater easy test though...go out and use the brakes really well...
then measure temps at each wheel...like each disc, and each drum.
I would expect temps to be roughly divided 60/40 ...( f, r ) something like
that ...hotter by that proportion in the front.
If you got way hot in the front, but kinda cool in the rear, or vise versa
..
that could tell you something.

there is the brake pressure limiting valve too ...
though I've never seen a bad one really. ..other than from corrosion.

here's another 'free test' that MIGHT shed some light.
Block off the vacuum to the booster ...
brake effort will be very high of course .....but see if the brake pedal
still has that same hit-something-hard feeling in it.

I'm leaning toward your MC so far. for 'leaks internally.'

is your problem more 'can't get a high pedal' or 'can't get good strong
braking action' ? ...or both ?

Scott