Bad Brake Booster?

The booster is doing exactly what it is supposed to do. After you shut the engine off, the booster hold enough vacuum for a couple of near normal stops. Once all the vacuum is used up, the pedal will get very hard.

You may need to pressure bleed the brakes to get the air out. Here's some help in that area...

Garden Sprayer Pressure Bake Bleeder

1 each pump type garden sprayer, 1 - 1 ½ gallon capacity (size doesn’t matter much, it just has to be cheap and small enough to be easy to work with)
6-10 feet 3/8” clear plastic tubing
1 each ¼” brass pipe tee
1 each ¼” pipe to 3/8: hose adapter
1 each pressure gauge 0-60 PSI or so – all you’ll ever need is 5- 10 PSI, so the gauge range doesn’t have to be high.
1 each large rubber stopper – this is the hard part to find. It may take some searching to find one that is a snug push fit to the inside of the filler port on the master cylinder. You can use silicone rubber to seal a brass fitting to an old master cylinder cap, but they tend to leak too much.
Home Depot or Lowes has some ¼” brass pipe stock that is continuously threaded in the electric lamp repair department, along with the brass nuts that go with it. This is better than a pipe nipple, since the nuts can be used to secure the pipe in the cap or stopper.
OR if you can’t find the threaded pipe stock,
1 each ¼” nipple, 1 ½”- 2” long

The rubber stopper needs a hole drilled in it for the ¼” pipe nipple or threaded pipe stock. After you drill the hole, use some silicone gasket sealer to seal the pipe nipple as you push it into the rubber stopper. If you used the threaded pipe stock, use the nuts to secure the stock into the stopper.

If you can’t find a suitable rubber stopper, an old master cylinder cap can be used. Drill a ½” hole in the center for the pipe fitting. Cut the brass pipe stock to about 1”- 1 ½” long, the exact length isn’t too important. Push it through the hole in the master cylinder cap and thread one nut on top of the pipe stock where it sticks though the cap. Put another nut on the other side of the cap to lock the pipe stock in place. Apply some silicone rubber gasket sealer to both sides and when it is dry, screw the ¼” pipe that sticks out of the top of the cap into the tee.

Remove the sprayer hose and wand from the garden sprayer. You may find it easier just to cut the sprayer hose off short and connect it to the 3/8” plastic tubing. The idea is that the 3/8” tubing connects to the pickup tube inside the sprayer in a reliable, leak proof fashion. Another alternative is to remove the spray nozzle from the end of the spray wand and connect the 3/8” tubing to the wand. This leaves the hand valve in place and may be useful to start/stop the flow of brake fluid.

The 3/8” plastic tubing connects to the pipe tee using a push on hose barb type adapter. The pipe tee has one port for the gauge, one for the 3/8” hose and the other to connect to the rubber stopper or master cylinder cap that you modified.

Fill the pump sprayer with a quart of brake fluid. Set the garden sprayer on the ground and screw the pump handle down tight, and pump until brake fluid fills the plastic tubing. Then put the modified stopper or master cylinder cap on the master cylinder and pump slowly to make sure that nothing leaks or pops loose. No leaks, continue pumping until you get 5-10 PSI. Put a 6”-12” length of clear plastic tubing on the bleeder ports. Then open the bleeder ports on the wheel cylinders one at a time and bleed until the bubbles are gone. I use a 2 liter soda bottle with a coat hanger to catch the fluid . DO NOT REUSE THE OLD BRAKE FLUID. Repeat the process until you have finished all 4 wheels. You will have to pump the sprayer several times to maintain the 5-10 PSI needed to do the job. When finished bleeding, loosen the pump handle to relieve the pressure, remove the stopper/modified master cylinder cap and test the pedal.

A bad booster will give the same pedal effort with or without the engine running. The engine vacuum pulling on a big diaphram is what gives the boost to the pedal pressure you apply.

You probably need to change the brake fluid and bleed the brake real good.