Front wheel locks up due to brake problem
I am currently using a Motobecane Mirage from the mid 70's (I think). It has Weinmann 610/Vanqueur 999 brakes. Before I was able to safely ride this old bike, I had to take it into the shop where the replaced the brake and shifting cables and installed new brake pads in the rear.
It worked very well for a while but now a new problem has arisen. Be forewarned that I don't know a lot about bikes so I might not be describing the problem accurately. On the brakes there is a sliding mechanism which moves in place when I apply pressure to the brakes. Look at the pictures below. In the rear, there is a red plastic piece which keeps the mechanism in place, but on the front, this cap has come off and the sliding piece is outside of the socket.
The problem with this is that when I turn front of the bike too sharply, the entire front wheel locks up and doesn't turn, which could be very dangerous. There is also some metal scraping off of the brake due to the misalignment.
If it makes a difference, the wheels say Chromage Superchromix, Rigida, 27x1¼ (HP), Made in France. The tires are Bell Streetsters.
At this point I have had such problems with these brakes that I wonder if I should get an entirely new system. If I do so, what do I look for in a new (but cheap) brake system? As in, what would fit this bike?
But, of course, if I could simply salvage what I have without having to spend the extra money, that would be great. Any suggestions?