Threebrakes keep pressure after pressing brakes.i disconnect power to the absit quits.plug it back in and they are normal until i apply the brakes then the same three brakes dont release completely.

1. Replace any hydraulic line showing external damage. Rubber hoses more than 4 years old should be replaced. Install new seals for all disconnected fittings (as required) and install an in-line power steering filter. Tighten all hose fittings to OE specifications. 2. Flush the entire power steering system using the vehicle manufacturers recommended fluid. Fill pump reservoir to the proper level.
3. Disable engine to allow cranking without starting. This can be done by unplugging the main power wire (large red one) from your Delco HEI Distributor unit. Block wheels, put transmission in neutral or park and set parking brake, then crank engine 5 to 10 seconds (avoid overheating starter motor).
4. Refill pump reservoir as necessary. Repeat step 3 until level is correct.
5. Enable the engine to allow starting. Start engine and let idle. Slowly turn steering wheel from lock to lock a number of times.
6. Turn engine off and inspect fluid level and condition. Add or remove fluid as necessary. If fluid is foaming, wait one hour then recheck level. Repeat step 5 and 6 until fluid level is correct and shows no sign of air. If the problem persists recheck installation and connections.
1. With engine off, apply and release brake pedal 5 to 10 applications to deplete accumulator reserve pressure. With foot slightly pressing brake pedal, start engine. Pedal should drop and then push back against your foot.
2. With engine running, turn steering wheel until lightly against steering stop. After 5 seconds return wheels to the straight ahead position and turn engine off. Apply and release brake pedal. There should be at least one power assisted brake application. Start engine and allow accumulator to recharge before moving vehicle. 3. Test drive vehicle.
NEVER operate any vehicle until brake and booster systems are functioning properly. Remember, your vehicle is only as good as it's braking system. Brakes are the singlemost important component on any motorized vehicle.

This sounds like the master cylinder has a problem with the check valves these are internal in the master cylinder. After removal of the master cylinder check to see if the rod coming out of power brake unit is threaded it may be to long and holding residual pressure in the lines keeping the front brakes applied. Testing for residual pressure is easy drive until locks up just turn bleeder screw without touching brake pedal if fluid sprays out fluid is not returning to master cylinder. good luck.

If the pads are being applied and you are not pushing on the pedal, it could be something going on with the ABS.

There is never a test or reason to pump your brakes

Accomplishes absolutely nothing.

There is no pressure in a braking system at all,
until you SLOWLY apply the brake pedal.

Then less than 1" off movement in the master cyl,
will develop 600 to 1800 lbs at the wheels

Release the brake pedal, the system goes to zero,
IT DOES NOT HOLD ANY PRESSURE

To answer your question

You not suppost to press the pedal hard
The saying goes--you apply the brakes,
not force the pedal arm and pin into the
vacuum booster, as though you want to damage it

When your driving what happens ?
You have higher manifold vacuum and
thus your pedal is normal,why the VACUUM
brake booster

Check that the pedal height isn't sagging due to low brake fluid level. Also check for adjustment of the brake light switch or the BPP (Brake Pedal Position) switch.

Brake Pedal Position (BPP) Switch Removal & Installation To Remove:

1. Disconnect battery negative cable.
2. Disconnect brake pedal position (BPP) switch electrical connector.
3. Remove self locking pin.
4. Remove switch from brake master cylinder push rod.
5. Remove push rod spacer.
6. Remove push rod bushing.

To Install:

1. Install push rod bushing.
2. Install push rod spacer.
3. Install switch to brake master cylinder push rod.
4. Install self locking pin.
5. Connect electrical connector.
6. Connect battery negative cable.

at the back of the pump where the high pressure line attaches. I'm not saying thats your problem, but thats where the valve is. I think if you disconnect it, it should have full boost.

Steering locking up is not normal. You have a hydro-boost assist power steering system on your vehicle. It uses the power steering pump to apply hydraullic pressure to assist in braking. The hydro-boost system may have a problem, pressure or restriction problem. Have dealer check booster and current updates to check valves, there may be a service bulletin for your concern.
Hope this helps.

Check for vacuum leak within power brake servo unit.
Remove vacuum hose from brake servo and block off the open vacuum line from the engine. Check that engine then runs normally when brakes applied.
Do not drive the vehicle with the vacuum servo disconnected, as the brakes may not be effective. - The pedal pressure required to apply the brakes effectively may exceed your capabilities.

If the brake booster servo diaphragm is leaking it will need to be replaced/repaired.

There is a sensor at the top of the rear differential for the ABS system. That sensor went bad and replacing it took care of the problem. What caused mine to go bad was apperantly water got into the differential when launching and loading my boat. check to see if gear lube needs to be replaced also.
The answer above is perfectly correct. When the ABS sensor on the diff goes bad the speedo no longer will work, the odometer stops recording mileage and both brake and ABS light comes on. You also loose O/D or "Lock-up" in the torque converter. The sensor is inexpensive and can be changed in about 30 mins. Good Luck!
I second the second answer :) My ABS light was on, my BRAKE light was on, my CHECK ENGINE light was on, my SPEEDOMETER wouldn't work below 35 mph, then quit working alltogether. Once the speedomenter quit working the odometer and cruise control quit working. I replaced the rear differential mounted speed sensor ($33) and then reset the CHECK ENGINE light by disconnecting the positive battery cable for a couple of minutes. EVERYTHING then worked.
Replace rear differential speed sensor as follows:
a) remove parking brake cable bracket (1/2" socket)
b) remove speed sensor bolt (9/16" deep socket)
c) remove metal speed sensor cover
d) remove speed sensor from rear differential housing (use channel lock pliers, twist gently back and forth while applying upward pressure. Once the speed sensor has a small gap between it and the rear differential housing, insert a flat-blade screwdriver and gently apply upward pressure while gently twisting speed sensor back and forth.
e) disconnect speed sensor from speed sensor cable
f) lubricate new sensor with WD-40, insert into rear differential housing. It will NOT go in all the way by hand, so do NOT force it nor hit it with anything made out of metal.
g) replace metal speed sensor cover
h) Insert speed sensor bolt and tighten LIGHTLY until a small amout of pressure is felt. Tap speed sensor with rubber hammer until it pops into housing. Finish tightening of speed sensor bolt.
i) replace brake cable bracket, replace 1/2" nut and tighten
j) disconnect positive battery cable for 2 minutes, reconnect positive battery cable
h) start vehicle, enjoy,

Don't wait until the last moment to put your brakes on. When you know you're going to have to stop, get off the gas and LIGHTLY, apply the brakes to start the slow down process. Putting a lot of pressure on the brakes to stop a vehicle causes them to wear out. I normally get about 80,000 from a set of brakes. I've been driving this way since 1970.