I have a '94 Honda Accord LX 4 cylinder with ABS. The brakes work fine until I press the ac compressor button, then they rapidly begin to fade, letting the pedal go to the floor. When I turn off the ac compressor, the pedal gradually firms up back to normal. If it was the booster, it would be hard to brake, but it’s not. If it was the master cylinder, the pedal would be soft all the time as the cups can not magically regenerate when the ac is off. What’s the connection with the ac compressor.
There is no connection the to the AC compressor. You have a brake problem. The AC compressor running, or not, is just confusing the issue. There is no electrical connection between the AC and brakes, no hydralyic connection, and no physical connection. You have a brake problem. That you notice a difference with the AC on is random chance.
Brake “fade” is something that happens on long hills and when you do a lot of prolonged heavy braking. What you are talking about is not brake fade, but a master cylinder not holding pressure. You might have a bad master cylinder, or a vacuum leak in the system the "powers’ the master cylinder. Neither is due to the AC compressor being on or off.
I wonder if the added heat from the A/C operation is causing the brake fluid to boil. This could happen if the brake fluid were never changed, and 17 years of moisture absorption and system corrosion has taken it’s toll.
i would not rule out the booster just yet. the booster operates on vacuum. if you have low vacuum at idle, possibly from a small leak the brake pedal could appears to operate normal. when you turn on the AC, the engine idle goes up, and so does the engine vacuum. have the vacuum supply line to the booster checked. also there should be a one way check valve in the supply line, to maintain some vacuum in the system in case of a collision.
If you open the hood, and look at the front left side of the engine compartment you’ll find the ABS pump reservior. Below this reservior runs both the low and high side refrigerant lines. Check the reservior for brake fluid level.
Disconnect the compressor clutch and test drive the car switching the AC on and off and braking. Also, assuming that it is an automatic, shift to neutral when braking with the AC on. As bluboy mentioned, when the AC is on the ECM pushes the idle RPM up to compensate and the IAC could be raising the RPM too high.