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I took the 5 in for a new muffler today, and once it was up on the hoist we discovered quite a bad coolant leak. It appeared to be coming from the water pump area. The water pump is only 20,000 k's old, but I'm suspecting the gasket due to something a bit dodgey I did that I won't go into right now. So once the new muff was fitted I topped the radiator up while watching it leak out under the water pump. I only had a ten min drive home so I took it easy, but temp was not rising above normal so I accelerated to highway speed and got home no worries. Here's where it gets weird. I pulled up in my garage expecting to see coolant leaking everywhere, but nothing. So I let it cool down & took the cap off the radiator only to find it full. Go figure??
I'm going to put a new water pump gasket on anyway, as well as checking all hoses etc. But why would the leak fix itself?

I'd be grateful for any ideas.

Cheers
Andrew.

Possibly water pump seal - these sometimes weep I am told, and take up again under pressure. I have a leak there, but it has not taken up.

I'm guessing you mean the internal seal, which means the WP is cactus, right? Geez, i'd hope it would last more than 20,000 Ks. If it is, I've got my old one still. It was working fine, I only replaced it because I was doing the timing belt. Any visible tell tale for a blown seal (apart from the leak)? I suppose if the gasket looks intact, it's pretty much got to be the seal?

Andrew, I have heard a few 86/BRZ owners commenting on coolant and the levels in the reservoir being considerably low

When I read that I went and checked mine in the garage, it hadn't been dríven for a few weeks and was just above the 'low' mark. I started the car and within 2 minutes it was just under full. Whilst it's a long reservoir and was hard to see at night, I was informed it should sit around half way to allow for rise and fall, some have filled to the top only to finds they have blown excess out of that container. Both my MX5's remain in the same position all the time

For the record the 86 had it's first service yesterday, all was good, no leaks etc and they confirmed what i'd been told - I guess the old 'expansion/contraction' thingy!

Hope you get that solved soon

Drive it for longer. Rev it higher. The leak will come back.

Yeah, well it was the day after the track day.
While doing some research I found this excellent description of how a WP works & what can cause a premature leak.
http://www.flowkoolerwaterpumps.com/info/common_waterpump_failures.html