Very recently the braking on my 1988 Suburban fades almost to nothing (pedal going right to the floor) whenever the air conditioning is operated for more than about 45 minutes. I suspected that the compressor was overheating and thus also overheating the brake master cylinder which is located just to the rear of the compressor. My local mechanic said "no" and spent almost $800 replacing all the pads/drum linings, front rotors etc. Still the same problem. Does it sound like an A/C compressor fault rather than a braking problem? When the air conditioning is not operated, the braking is just fine.
Many thanks for any help.

 

Welcome to the forum.

Does that model use the Hydroboost power brake system? It uses hydraulic pressure from the power steering pump for the PB booster instead of vacuum. It's what GM used on diesel trucks.

If it does and the PS pump is using the same belt as the A/C, then I suspect belt slippage when the A/C is on.