When i start van each morning and go to leave the first time i hit brakes they lock up or grab very hard. pedal pumps up and then are fine. whats is wrong 1997 gmc pass. van 170.000 mi. v8.

check for crushed brake line

I would hope they lock up when you hit them hard!

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

If the ABS system were the problem, the ABS warning light should come on.
So I would say the problem is with the vac booster. The booster is between the pedal and the master cylinder. It uses vac to boost the pressure applied to the master cylinder by your foot.
You may have to leave the vehicle with a mechanic to drive until the problem occurs.

Start with brake in the rear that is the farthest away from the master cylinder ( if your master cylinder is on drivers side start with passenger rear,pump up brake pedal until hard have someone else open bleeder valve until air or fluid squirts out,while holding brake pressure even if goes to the floor dont release pedal until bleeder valve is tightened or you will **** air back into the line,do this several times making sure you get a full stream of brake fluid and no air,continue to other rear wheel and then proceed to front wheel furthest from master cylinderand then final wheel. if you still don't have brakes change master cylinder and bleed all brakes again.

THE MASTER CYLINDER NEED TO BE BLED FIRST.BEFORE PUTTING ON THE CAR.TO MANUAL BLEED BRAKE SYSTEM.YOU NEED 2 PEOPLE TO DO A BETTER JOB.YOU FIRST BLEED THE LEFT REAR RIGHT REAR LEFT FRONT AND RIGHT FRONT. WHEN YOU BLEED THE BRAKES.HAVE THE HELPER PUMP BRAKES 3 PUMPS.AND HOLD BRAKE PEDAL IN.YOU NEED A CLEAR BLEEDER HOSE IN A CLEAR BOTTLE HALF WITH CLEAN BRAKE FLUID. WITH BLEEDER HOSE ON BLEEDER SCREW AND OTHER END BLEEDER HOSE IN BOTTLE OF BRAKE FLUID.WHILE HELPER PUMP BRAKE PEDAL HOLD IT IN OPEN BLEEDER SCREW 1/4 TURN ALLOWING AIR BUBBLES LEAVE THE BRAKE SYSTEM.CLOSE THE BLEEDER SCREW BEFORE BRAKE PEDAL RELEASE.HAVE HELPER PUMP BRAKE PEDAL 3 TIMES. WITH PEDAL HELD DOWN OPEN BLEEDER SCREW GETTING AIR OUT OF THE SYSTEM.WHEN YOU SEE SOLID FLUID COMING OUT OF BLEEDER HOSE THE AIR IS OUT THE BRAKE SYSTEM.DONT DRIVE CAR ON HIGHWAY IF YOU STILL CANT GET SOLID BRAKES. HAVE CAR TOWED IN TO BE PRESSURE BLEED.

sounds like you have a bad vacuum booster on your brakes or their is a severe systen vacuum leak

sounds like the actual booster...can you pump the pedal several times while it is off and then start it with foot on pedal and feel any difference?

When was the last time you changed the front brake pads?
Try this:
Keep the brake pedal pressed down and start the car, after it idles for about 10 second let go of the pedal. If the light comes on just as you leave the pedal - try pumping it a few times, turn the car off and pump the pedal some more until it becomes real hard.
Then, keeping your foot to the pedal, start the car.
Idle for about 10 seconds, let go of the pedal and press it again rel fast.
If the light doesnt come on again - it means you have a leak of some sort or that the main brake pump should be replaced.

Good luck.

Had a similar problem a number of years back. Underneath the brake pedal arm is a small switch with a plunger. when you press the pedal down it moves the plunger in and engages the brake lights. when you let off it's supposed to pop back out (turning the lights back off). Possible it is getting stuck in the down position and could need to be replaced. very easy to install and only about an 8 dollar fix.

you could have a brake booster issue. you can do a simple test on another car by leaving the engine off and press the brake repeatedly 3 or four times will use up the vacuum in the brake booster. Once you have done this does it feel like the brake pedal in your van?