Pressure testing.

Getter closer to having the van running after a HG change and skim, in the process I've used the method posted here to pressure test the cooling system: Schrader valve in a bit of tube on a Tee and up to 25psi. As a result the system no longer has any leaks - save one. With the overflow pipe clamped there is no leakage, but if I release the clamp, the pressure slowly drops to 0psi. The cap is a new "Gates" rated at the usual 0.9bar, the sealing surfaces in the metal pipe have been cleaned up. There was a small amount of pitting in the "inner" surface so I've used a little chemical metal to fill the pits,sanding it smooth with some wet & dry paper.

My Q is: I presume the interface between the inner seal and pipe isn't air tight, can I safely assume that coolant will act differently and the cap will in fact hold the wet stuff in rather than blow it out into the expansion tank. Or to put it another way, when pressure testing using the above method, does the system bleed down like this anyway?

Ta in advance.

Yes it does - at first the air seams to leak away quickly down to about 7 psi, then very slowly from there.

Some caps are better than others, and it you cover them in a load of vaseline then tend to hold air much better.

Thanks for confirming.  Looks like I'll be filling the cooling system tomorrow then.

In other news, while looking about on on ebay I found this:

32mm inline rad filler neck and 0.9bar cap

My thinking is if the original filler neck is too far gone, instead of throwing out the entire pipe, chop out the offending section and weld this in.  Make do and mend.

Normally if the cap seat is buggered, the whole pipe tends to have pitted rust points all over.

Normally if this pipe is buggered, someone has been pouring just water into the coolant system for quite some time.