The ABS light comes on intermitenly for the last few weeks. Most of the time it blinks when going over pumps or at high speeds, 60-75. I checked the lines and bounced the front of the car when ideling to see if I could make the light come on. No success. Recently when the light is on or imediately after it turns off and I hit the brake it begins to stop then the pedal sinks a few more inches and car surges forward to eventually stop. A minor grinding or gurling noise occurs when the pedal sinks. The rotors are new and smooth, brake pads fine. Is it the master cyclinder? How hard is it to change?

Sounds like the system is 'falsing' - detecting wheel slippage when none really exists.

This is most likely related to a flaky front wheel sensor. he 3 channel system sues the speedometer electronic pickup on the rear diff, and the speedo would be acting wonky if it was having issues.

The sensors on the front wheel can have various issues.
1) Rusty rotors - they have a toothed section on the inside back edge. If they get too crysty/rrusty the on/off/on/off saw signal it is supposed to generate on the sensor could get fuzzy, and if the sensor cna't pick out a tooth it just asusmes it wasn't there, and that the wheel is going slower than it is (sliding) and releases pressure.

2) Sensor clogged with metal particles - the sensor is magnetic, and metal shavings from the rotor and rust particles can and will cling to it. If enough gets on it, those shavings will cloud and overwhelme the signal from the toothed rotor.

3) Wiring faults - anywhere between the sensor and the ABS computer, although most of the time inside the wheel well, possibly in the connector in the engine compartment (follow them through the inner fender, they have a connecto near the firewall).

4) Plain old bad sensor - sometimes they jsut go bad, and often when they do vibration (like bumps) sets them off and falses the ABS system.

The grinding and gurgling is most likely just the proper ABS activation sound. When ABS is activated, it will bleed hydraulic pressure from the channel it senses is skidding, that's the buzzing sensation. If that's all it did, then the pedal would sink to the floor, which would be bad. So they have a pump that also activates to 'refill' the master cylinder - supposedly replacing the fluid bled to unlock the wheel. But it isn't an exact science, the pedal generally sinks somewhat. If it doesn't go very far, it's probably not a problem. If it sinks to the floor, the pump is probably not working well.

And, as a final 'round tuit' reminder - flush fluid every 3 or 4 years. Brake fluid absorbs moisture, which both lowers the boiling point (boiling brake fluid in a caliper means no brakes) and increases the chances that rust will form internally. Every few years, bleed the brakes at all 4 corners until fresh fluid comes out.
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I'm JohnMc, and I approved this message.

Thanks. I'll flush, bleed and check for wear or buidl up this weekend.

I like to use ATE Super Blue fluid for a flush. It's about $9/liter at FCP Groton, but meets or exceeds DOT 4 specs. And for a job so worthwhile but so seldom done, what's another $5 or so?

What I like most is seeing the blue color come out the bleeder—telling me that line is flushed thoroughly and avoiding wasted fluid. The 1-liter does the non-ABS 240 with some left over—so it's more than enough for your ABS.

Make sure you can get fluid from ALL the bleeders before you start. Finding a problem part way thru takes a lot of the fun out of it.
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Bruce Young
'93 940-NA (current), 240s (one V8), 140s, 122s, since '63.

Just to clarify, the flush/bleed won't help this issue, it's just a good preventative maintenance.
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I'm JohnMc, and I approved this message.