sure, you can just bleed the rears after working on the rear brakes.
it's not a thorough a job as bleeding in some new brake fluid at the front
end too at the same time.......but you'll be all right.  I bleed just one
wheel many times......like if I do anything to one wheel or brake, I'll
usually bleed just that wheel if the brake fluid has been in there a while.
The more you change brake fluid ........the longer the hydraulic brake
components last - simple as that.

the only reason I can think of not to change any brake fluid is you know or
suspect a hydraulic component is one the edge of failing.
Changing brake fluid on any part that's close to the end of it's life can
cause it blow out for sure . Other than that....new brake fluid is always
better ......as long as it's a quality brand.  Brake fluid absorbs moisture
out of the air, just sitting there even. Every two years for all cars -
change the brake fluid.
Scott
www.turbovans.com

----- Original Message -----
From: "Troy" <colorworks@GCI.NET>
To: <vanagon@GERRY.VANAGON.COM>
Sent: Friday, May 22, 2009 3:24 PM
Subject: a quick question about bleeding brakes

>I have an 89 Westy. Are the front and rear brakes on separate circuits so
>that I only need to bleed the rears after replacing some wheel cylinders?
>It's not a major deal to do the fronts as well, but if I don't need to I
>just as soon not mess with it.
>
> Thanks,
>
> Troy