Thanks for the post!

First off, what is the model# of the unit? This will help in diagnosis.

It sounds like there may be more than an issue with the chemical injector here. The soap feeder tube usually hooks to a small fitting on the high pressure outlet and will only siphon chemicals or soap at low pressure through the spray gun. There is usually a check valve in the chemical injector that prevents water from leaking back through the soap tube. The check valve is usually a ball with seat and a spring, there is usually an orifice made into the housing. If the ball, seat or spring were damaged or seized from mineral buildup this could cause it to leak back through the soap tube.
It also sounds as though the unloader is seized in the aluminum pump housing; this is a common occurrence.

When you say that you get 'some pressure', can you clarify a bit? Does the unit have the proper pressure and just leaks water through the soap tube or do you think that the soap tube leak is causing the unit to lose pressure? OEM pumps are usually good for about 50 hours of use, if there is a high mineral content to the water or no filter is used then the pump life will be cut. Based on the information that is available here I would say that this unit is going to possibly need a pump.

- MTR

Not sure if you mean the valve in the ground or under the house?

Depends if you have a gate valve, and that's what it sounds like.

If so, then the packing around the handle can be replaced in most cases, but the water has to be turned off at the street.

This is a gate valve. If you remove the screw holding the red handle on, then remove the top gland nut, there is usually packing material below the gland nut. You can purchase the packing material at a hardware store, wind it clockwise around the stem, then tighten the gland nut, but not too tight. This should fix it.

Hope this helps a little!