Brake booster repair question.

Toyota recommends you replace the two units as a pair how are you going to diagnose what's causing the constant whirring pump. There's a leak internally somewhere and the pump is constantly pumping to pump the accumulator back up to spec then it leaks down into 10 seconds you're talking about and whirrs again You could replace just the one unit but then if you're mistaken on your diagnosis you have to take it all apart again and replace the other piece You can risk that or not if you're very good with a mechanic stethoscope you may be able to isolate it to the one unit in other words figure out where the leaking is taking place down there in the lower unit or the one above with the plugs and all the lines on it If you're real good you might be able to separate it with the stethoscope and figure out which pieces actually bleeding down and then replace it But if it doesn't work then you'll have to replace the other piece either way it seems to be about the same cost on a generation two there's only one piece to replace it's around $1,400 bucks and the generation 3 they split it into two pieces that are about 700 bucks a piece or about 1,400 for the total system the two pieces even Toyota recommends replacing them as a pair so you don't wind up with double the labor and another part to purchase I guess you can take that recommendation with the grain of salt with this model I would not do that.