Disconnect all the lines going to the main vac line, and hook your vac gauge to the line. If there are any other T's in the line plug them. Then remove the main vac line from the brake booster and plug the end. Then start the car and see what readings you get. If you get less than 18-20 your pump is in need of some attention. It shold also hold stead, no jumping around. If it hold stead with good vac, then you have a leak somewhere else. I'd screw the line back onto the booster and read it again. The booster could be leaking.

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