Coolant bled issue

[Tsporter]

So I've changed the thermostat and flushed and bled the system. I've got to the point in bleeding(with the rear raised) where the level in the rad cap doesn't drop. So I thought that was it. But I took her for a long drive and left her overnight. Now, she is warming up and idling perfect but the next morning the level had dropped under the rad cap and I had to top up with about 300mls coolant. I tried bleeding again and got to the same point where the level doesn't drop. I thought ok, but then the same thing happened after a long drive.
The coolant seems to be going into the expansion tank as the level in there is rising about the amount I'm topping up...

Any ideas?

I've ordered a new rad cap just in case that's faulty, but I'm only guessing.

I've a trackday on Friday so want to get it sorted.

[PW@Woodsport]

Pressure cap is one possible, also the O rings on the thermostat housing bleed valve may have gone, both will allow coolant to reach the expansion bottle.

A simple test for the pressure cap is to get the engine hot, take off the pipe that runs from coolant filler neck to expansion cap and rev the engine hard, no coolant should come out of the neck pipe, if it does then you either have a faulty cap or excess pressure in the system that is overcoming the 0.9bar cap pressure, usually caused by head gasket failure.

Check those O rings on the bleed though, it's usually them.

[Tsporter]

Thanks Paul!

I've ordered a new pressure cap which should arrive tomorrow, so that will narrow that down anyway.
Is there any way of ruling out the thermostat bleed valve o-rings? Or will I just order and replace them? ....they are cheap...

I also have the old white expansion cap which I believe causes problems so I'll get one from somewhere.... Will a mk2 one fit?

[BIGBOBO]

The EPC lists 6 and 4mm ID for those O-rings on the bleed screw.

[Tsporter]

Great thanks, ill hit the local motor factors in the morning.

[PW@Woodsport]

Thanks Paul!

I've ordered a new pressure cap which should arrive tomorrow, so that will narrow that down anyway.
Is there any way of ruling out the thermostat bleed valve o-rings? Or will I just order and replace them? ....they are cheap...

I also have the old white expansion cap which I believe causes problems so I'll get one from somewhere.... Will a mk2 one fit?

Yes all of those plastic caps are the same on most Toyotas.