I have an occasional hard brake pedal so I ordered a brake check valve. I took off the old one and blew air through it and air went through both sides. So I thought this must be the problem but then I blew air through the new one and it does the same thing. Looking inside them I see no valve apparatus. So is it really a check valve?

If you can blow through in both directions, it isn't a check valve. Is this what you have? https://www.fcpeuro.com/products/volvo-brake-booster-vacuum-connector-s60-v70-xc70-xc90-9473812

Even a dealer shouldn't charge more than $10 for it.

I thought your car also has a pump which can become defective and lead to erratic pedal pressure.
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Keeping it running is better than buying new

Thanks for the tip about the pump. I hear the pump on my 2008 C70 when I turn off the engine but not on the XC70, so maybe that might be it. If the pump is working correctly should I hear it then?

The 2008 C70 and the 2007 XC have the same pump. You could swap it for a test. These things are expensive!

Also check the check valve on the C70.
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Keeping it running is better than buying new

When I went to identify the pump and switch, I touched the vacuum pump switch and it popped apart. It has a spring inside. The plastic tabs had deteriorated. So I may have identified the problem. I ordered a new switch since it needs to be replaced whether or not it is the root cause or not.

Yes, that is the Volvo part number on the package. The fcpeuro title for it says "check valve". But there ain't no valve, just a 90 degree connector. On the package the description is "L-Connection".