I Hanve 1996 GMC 1500 brake pedal goes to floor. New brakes & Calipers in front, 1 new wheel cylinder. 3 New master cylinders,1 new brake booster, Brake line in rear rusted out & replaced. Have checked rubber lines for collasping or bubbling, When Bleeding all wheels lock, Have vacum bled and manual bled. Tried different ideas, do not touch brake until bleeder is open and depress and close bleeder. Tried just about everything, Have adjusted rear brakes out. When truck not running you can pump up brakes, when you start the truck the brakes will go to floor, will not pump up when running.

You have air trapped in the braking system somewhere. Have you bled the system thoroughly at all four wheels?
Have you changed the front calipers recently. Many years ago, I helped a friend mount new calipers on his vehicle, and we put the new calipers on the wrong side -- right one on the left side and vice versa. In this configuration, the bleeder screw wound up on the lower portion of the caliper instead of at the top, where it needs to be to allow trapped air to come out. Just a thought.

You describe a brake problem of a master cylinder leak.
But the vacuum booster could interfere with the application of brake pressure on the master cylinder.
For a firm brake pedal and holding a leak inside the brake booster.
and rotor, front pad, drums, and brake band have to be in very good condition.

Hope it helps.

Michael:

You must start bleeding the brakes at the wheel farthest from the master cylinder (usually the right rear), then the next farthest from the master cylinder, then the next, then the closest. If your master cylinder is at the left front of the car, start with the right rear, then the left rear, then the right front, then the left front. If you don't bleed the brakes in the correct order, you are just shifting the air in the lines from one line to another. Make sure that you close the bleeder before letting the brake pedal up, and the engine should not be running when you bleed the brakes... Make sure that the emergency brake is off. Make sure that the master cylinder does not run out of brake fluid at any time that you are bleeding the brakes.

check the master cylinder. it can go bad without leaking. the fluid bypasses the internal cups causing a fall in the pedal. also if you broke the lines lose to do calipers you might have air in the system bleed it good. if not i suspect the master cylinder

Check all the calipers and wheel cylinders

check for front brake calipers dragging, if the are replace calipers and front brake hoses, if calipers ok replace master cylinder, This is how to check calipers, jack up front of truck have some one apply brakes and release as soon as they release the brake peddle the wheels should spin freely if not brake the brake line lose at master cylinder if wheels spin free with line loose replace master cylinder, if not replace calipers and hoses.....HOPE THIS HELPS

check vacumn and\or connections to the vacumn boost cannister.

1. Check brake fluid level and look for leaks in the brake lines and at the calipers.
2. Bleed the brake fluid completely and replace with fresh fluid (google search: how to bleed brakes). Then bleed air by having someone press the brake pedal 4-5 times, then press and hold while you open the bleed valve and close when fluid streams out. start with the rear wheel (if 4wheel disc brakes) then move to the front.
3. Replace Master Cylinder (sorry, but if 1 or 2 didn't fix it, this is the last option) This is usually over $150, and will involve step 2 again when completed.

Good luck, hope this helps.

no they are inverted flare fittings no seals check all lines and calipers/wheel cylinders if all are dry remove the vacuum hose from the booster if there is brake fluid in the hose the master cylinder is bad

Did you bench bleed the master cyl. before installing it? then when you installed the new master, did you bleed all 4 points? If yes, then check all for points for leaking caliper/s and leaking wheel cylinder/s. If the van has ABS, check the valve body for a bleeder screw and see if air bleeds out of it. If you've checked everything twice and did everything right then your new master cyl. may be defective.