While sitting at a traffic light the brake pedal slowly goes down. One pump and it comes back up to normal and holds. This is a '95 LeSabre very low mileage (31,000) very clean car. No signs of brake fluid leaking anywhere. Help?

does the pedal have pressure then go down after valve is cracked? With a hose on the nipple to a bottle to catch the fluid, press rear brake pedal down. Don\'t pump the brakes. Crack the bleeder valve while continuing to press down on the pedal. Once the pedal reaches the limit of its travel close the valve. Once the valve is closed; and not before; release the pedal and let it return to its at rest position. Repeat this process until you have good pedal without the need to pump, good clean fluid is coming out the valve, and no air is in the fluid coming out the valve. Be sure not to let the fluid in reservoir get too low or you will suck in air and need to start over and don\'t press the pedal too quickly or you can shoot a stream of brake fluid out the reservoir.

In very extreme cases, if you have lost prime in the master cylinder, it may be necessary to bleed the master cylinder first. THIS IS A LAST RESORT. Remove the brake line from the master cylinder. Try to limit the loss of fluid from the brake line to bare minimum. Thread a nipple into the master cylinder. Run a hose from the nipple into the reservoir. Be sure the hose stays submerged in the fluid in the reservoir and pump the brake until no bubbles come out the hose. Remove the nipple and reinstall the brake line. Slowly press down the brake pedal while tightening the line to minimize the air bubble in the brake line. Again be sure not to release the pedal until the line is tight. Then proceed to bleed the brakes as you normally would. Do not pump the brakes up. Just press the pedal down, crack the valve, close the valve, release the pedal, repeat.

This takes 2 people master cylinder is full lid back on . have assistant pump brake pedal 3 times then hold down saying "holding" then you loosen bleeder screw on right rear brake slowly air will come out then a little fluid. tighten screw and say pump it up "holding" as fluid in coming out pedal will sink "Keep pressing" repeat util clean fluid appears. check master fluid level now repeat left rear. fluid level right front fluid level left front fluid level once complte you should have a firm pedal.

If you step on the brake pedal and hold it down and you feel the pedal going slowly down, that's for sure your Master Cylinder unless you have a brake fluid leak from one of the calipers. If you have a brake fluid leak, you will notice it because you will have to keep on adding fluid to the system. If you notice a change in your RPM when you brake then you may have an air leak from your vacuum to manifold hose.

sounds like the brake master cylinder seals are worn buy a master cylinder kit and get a mechanic to repair it it is a lot cheaper than replacing the whole unit

When engine is running you have a power booster in operation. Without power booster brake pedal will be heavy. With engine running, do NOT press pedal really hard. Just normal firmness. If pedal goes to floor or just too low, it needs bleeding.

Are you sure you did not run the resovior dry?
I would think would not hurt to check the correct procedure for bleeding rear brakes. Here is a YouTube video which should help. http://www.youtube.com/watch?v=8T7Qrz3zJno

The other possible issue is the master cylinder piston is not sealing well. Might be a good idea to flush it with fresh fluid, but I imagine you have already if you bled it 4 times.

The way I was taught is this. Two persons method. To prevent mess, put a flexible tube over the nipple and drain it to a jar.
Fill resovior with fresh brake fluid. get ready at the bleed nipple with a small closed end wrench, and have helper sit in driver seat. Have helper pump brake pedal (all the way to the floor if it goes there without forcing it) 5 - 8 times, then HOLD PEDAL DOWN. Slowly open the bleed nipple and close it again BEFORE the pedal is released. Helper should keep pressure on as pedal moves down when fluid is released. Repeat until there is no sputtering or bubbles indicating air comming from nipple. Check and fill the tank every couple times to prevent emptying. If it run dry, you have a big problem. You must bleed the whole system at every wheel till ALL air is out.
Hope I have helped.

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

There could be any number of things wrong from air in the lines to a leak of fluid. In any case you need to get the car to a mechanic as it loss of brakes is very dangerous.

un plug the module under the master cylinder and see if brake lamp goes out...if light goes out you may have a rear abs problem..hope this helps you!!

bad wheel cyclinders, or master cyclinder, possiable fluid leak

Ok the BRAKE BOOSTER is ONLY there to give you pwer assisted, otherwise NO POWER BRAKES. ( not anything to do with the low brake pedal ) Hopefully the junkyard master cylinder was in better shape than your original. AS far a bleedinda system, heres what I do. First with a1998 vehicle, I would go to ALL 4 wheels , OPEN & CLOSE slightly each bleeder to insure none of them are FROZEN, and they do freeze shut ENSUE THE MASTER CYLINDER IS FULL and CLOSED Now with an assistant sitting in the drivers seat, ready to pump the brakes, and ABLE TO HEAR YOU. Go to the furthest wheel from the master which would be the R/R. DONT touch anything yet. Tell the asst too slowly pump the brake pedal till it gets hard, and then HOLD THE PEDAL DOWN ( and dont let up on it until the pedal drops. If the ast lets up before the bleeder is closed you **** in air.Repeat at each wheel til the pedal stays hard. Hope this helps