Low pressure in sink's cold water supply

The cold water supply pipe under the sink in the bathroom in our house is clogged. For a long while the old sink's handle would not turn and it would only supply hot water. When I replaced the old sink with a new one due to a leak in the shower hose, the cold water pressure was extremely weak in this one place.

Today I tried pushing the water out of the pipes with an air compressor. I had a very small amount of success in that after I reconnected the pipe, something which looked like the water scale seen in other pipes seemed to be mixed in to the water. However the water pressure is still very weak.

My next idea is to try taking off the stop valve, blowing out some of the water with the air compressor again, and dropping some citric acid into the pipe to dissolve the scale, then leaving it for a few hours before draining the system. I think the pipes are made from stainless steel so this should be reasonably safe. I've used citric acid successfully to remove this scale from the toilet bowls.

This photo shows the cold water with the tap fully open:

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As you can see the water pressure is very low.

This photo shows the cold water stop valve under the sink:

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My plan is to remove the above valve (the one with the label still attached), blow the water out with the air compressor, then drop a few grams of citric acid in there.

Here is a bendy pipe from a kitchen tap which I replaced, showing scale buildup:

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The colour of the water after blowing out the water through the above valve was quite similar to this scale's colour, leading me to believe that the pipe is clogged with scale from the ten or so years that we couldn't use the cold water in the bathroom sink.

Update 21 December 2016

I tried again to clean the pipe with the air compressor.

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This time I was able to blow the cold water out of the system:

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I also checked that the problem was with the water pipe and not with the new sink. As can be seen from the following photograph, the water pressure is very low

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I also tried putting the citric acid in the system. Unfortunately a bit of the tape fell into the pipe and I ended up having to fish the tape out of the pipe with chopsticks. Fortunately I was able to retrieve it.

Update 25 December 2016

I tried running water from a bathtub back through the pipe with a pump called "Minipondy". Here is the pump part in the bathtub:

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Here is the connector I bought from D2, and the hose connector is from Daiso (100 yen shop):

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Here is the pipe connected up to the pump:

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That didn't affect the flow, and only a trickle could be produced.

I also tried putting citric acid in the pipe directly and leaving it in the pipe for a few hours. That did something, which was to make the water turn an interesting pale blue colour. The colour seems to be from copper citrate, so the acid has dissolved some copper from either a brass fitting or a copper pipe. Since copper solutions are not a healthy addition to the drinking water supply, and burning holes in the already-corroded pipes or fittings with acid is not going to help, I won't be trying that again.

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