Thats the first I've heard about this - its a pressurised system with a one way bleed valve which only lets air (or fluid under high pressure) out.

You're correct and that prompted me to go through my notes. I had the coolant tank replaced two years ago. and in looking that up it turns out I've had this sudden drop/bubble before, the first time I did a coolant flush after the car was 10 years old, or a year before the old coolant tank split. Seems like each time the cap is taken off the coolant tank, I end up with a drop in coolant some time later. First time was the flush and recently with the waterpump. I still have the car's original 2000 cap....

I'm not sure if this is a factor related to the cap issue, but both drops in coolant seemed to occurr when ambient air was hot (over 90F) and I was driving the car around for at least an hour.

Perfectlap: the coolant went just below the low mark and then I had it topped up. This occurred in a few days and hasn't moved since.

Hmmm, I ahven't had to top up my coolant since I DIY'ed the pump and both rads 30k miles ago.

I didn't do anything clever, but I did follow the bleeding routine from (I think) Pelican Parts to the letter. Sure it's a little long winded, but it's not that bad.

Can only go on my experience and my experience is that you don't need fancy equipment.

Water pump questions ?
 

Let me start by saying [B]thank you[/B] to all here who have contributed information to get me this far. I had a water pump failure in the best possible place. Three blocks from home. I heard a thump and the warnings lit up. I thought it was just the belt but inspection revealed the water pump pulley shaft had snapped. OK I got 83,500 K out of it. When I removed the pump the original full gasket was present, but there were newer hose clamps and possibly at least one new hose at the pump. I have no records from the PO (grrr). I don't know what type of coolant was in there. I got four gallons of green stuff out. My first question is. Was the original coolant green? I plan to flush but don't know how meticulous to be as I will be using Porsche pink and distilled H2O as I go back to avoid [I]the gel[/I].Second question. Are the longer water pump bolts located at 3 and 6 o'clock looking in from the passenger compartment? They seem well seated in those positions. From advice here I installed a composite impeller pump, new low temp thermostat,new hose clamps and drain plug crush washer. I inspected the engine mount and did a new fuel filter since I was there. Bought the latest version of the coolant expansion tank cap. Got a new serp belt although the old one looked OK. I also did new mobile 1and filter, repaired under panels (thanks Pedro) replaced missing bolts on brackets and threw in an air filter. I can see upcoming service needs and will address the usual suspects as this experience has been rewarding thus far.

I feel for you, man. Been there, done that. What a PITA. I had to drain it all and flush numerous times.

From my recollection of the issue it could be your cap. Why not replace it with the updated version - they're cheap.

Vista glass, I did this myself. Had green went pink. :) I just bought 10 gallons of distilled water for the flush. After the first one, the second came out mostly black (in the bottles). I bought another 5 gallons and flushed it again. Seemed to be hose particles. Which is why I replaced my main hoses and will repalce all the front hoses when I add my center radiator this fall.

It is a pain to fill and drain while still on stands and there may be an air bubble or two. I always had to do multiple fills during the heat cycle venting procedure as descrribed in the bently manual. I bought the uView 550000 because of that and used it for my final fill. When I have to flush it again - will for center radiator if nothing else - I'll use the uView to make my life easier for the flush and fill cycles.

re: water pump failures
 

I've had my 2004 Boxster S a few years now and replaced all fluids including Porsche-branded ($35/gal) coolant.
It has about 40k miles on it now and I want to run it up to the Montreal GP weekend (8 hrs from PA).
My concern is: 15 year-old hoses and water pump survival for 16 hours on the road round trip.
How many hoses are there, anyway??
Opinions?

Not trying to incite a riot but I have replaced my water pump twice and added an S center radiator on my 99. Used the burp method each time never had a problem.

Before I changed WP, Themo and coolant flush, I drove several multi day 400+ miles/day road trips on the original WP with 85k+ miles
I drove 1000 miles straight on that same WP to Fla. once as well