It's a little difficult to say exactly what the problem is with the info given, but you may want to take the head off of the pump again and make sure you didn't damage the Compression Ring when you installed it. It's easy to do, all it takes is a small tear or rip and you'll loose compression.

Also, check to make sure the Belt isn't slipping; make sure it's on tight and that it's not worn out, this can be a pretty common problem with this type of compressor.

The Valves COULD be defective, but if they appear to be okay and not rusted or cracked, they should be fine.

I've also attached a Trouble-shooting document to this post for your reference.

I hope this helps!

- MTR