abs brake problem

I have a 1994 gmc 2500 van with absand hydro assist brakes. Under normal driving conditions it stops just fine ,but when you have to make an emergency stop it goes crazy ! It slides,makes grouling noises and even kills the enging sometimes . A second stomp of the pedal it went almost to the floor but did finally stop. this happened twice when my wife was driving as this is her vehicle . I took it out and re simulated the stop and sure enough it happened . after I re started the engine the abs light and the brake warning light was on , after backing up and pulling foreward and normally appling the brakes the light both went out and everything was fine again .when sitting in the driveway and appling extra pressure to the brake pedal there is agrouling noise which I assume is probably normal for a hydroboost system. Any information will be appreciated. thank you.

Needs to be driven and scanned with an abs capable scanner that will read codes and look at individual wheel sensor inputs.