'98 F150 brake issue

Got a brake question for everybody. (’98 F150 v8 Lariat, 140 k miles) At very low speeds, like pulling into a parking spot or approaching a stop sign, the brake pedal suddenly becomes very hard and the truck doesn’t want to stop. It’s almost like the anti-locks are kicking in. The weird part is that if you really “stand” on the brakes the truck will suddenly stop. We bled the brakes yesterday, thinking that there was air in the lines. We (maybe) saw a little air, no fix. This doesn’t always happen, just sometimes. I took it out for a drive around the neighborhood today and made lots of slow speed, right hand turns. Happened about 1/3 of the time. Doesn’t seem to be any problem with “high speed” braking, just at low speeds. But then again, I haven’t had any “panic” stopping situations with this truck. The anti-lock light has come on a time or two, but there doesn’t seem to be a direct relationship to the problem at hand.

I read in another thread with a similar problem that the anti-lock sensor may be dirty or bad. Where is this sensor and is there a way to test it directly? How would you clean it?

Thanks in advance for all of the help.

Jim

Check your vacuum line going to the vacuum brake booster and/or check the vacuum booster for a leak.