It is likely a tired internally leaking MC.
other things to check...

front pads are good,
caliper pistons are not stuck.
caliper 'slide pins'  are not stuck/jammed/rusted.
( give some thought to the front wheel bearings ...amazingly ..they
often go with no service or even inspection for decades on 2WD vanagons
...but 'something' could/should be done about those too. )

rear wheel cylinders not leaking.
rear drums and shoes are good
rear wheel bearings do not have excessive play

brake lines are not rusted,
brakes hoses not too old and cracked
brake proportioning valve is not desperately ancient and rusted
and all brake fluid is fairly clean and fresh.

It is possible to replace the MC and not bleed the BF out to all wheels..
but one really should ..bleed to all wheels.

and freshen up the clutch hydraulic fluid while you're at it, if manual
trans.

it can go on and on if one is really thorough.

oh yes !
check for brake fluid in the Booster ..
there shouldn't be any really.
it's 'not good' if there is.
mind that o-ring where the MC bolts to the booster ..make sure that's in
place.

what fun,
Scott
www.turbovans.com

On 3/19/2012 12:09 PM, Aaron wrote:
> Hello All.
>
> I was noticing the other day that the brakes seemed a little softer than usual. While playing with them, I noticed that when I had put some pressure the brake pedal while stopped, it would slowly sink, even though I was not changing the amount of pressure applied. I don't appear to be losing fluid. After a little archive research, it looks like it's likely a master cylinder.
>
> Anything else I should look at?
>
> Thanks!
> Aaron
> '87 Westfalia