My son was driving his '90 240 with 220K miles when he says the brakes suddenly stopped working. He said they felt "odd" at one stop and at the next red light the pedal went to the metal. Fortunately, he was able to stop with the e brake. When I went to check it, the brake pedal felt nice and firm, but as soon as I started the motor, the pedal sunk to the floor. I removed and sealed off the vacuum hose to the booster and was able to drive home (with the expected hard/poor braking). There is vacuum at the check valve, and the check valve seems OK (air flows towards the manifold, but not the other way.)

I assume the problem is a bad booster, but I've never heard of any failing this way. I thought that when they failed, they simply stopped giving a boost, and you could still rely on the brakes working. Before I plunk down the $$$ (and put all the effort into removing those 4 nuts), is there an alternative diagnosis?