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Any brake system will harden with 3 or 4 pumps. The key is how soft they are to start. Replacing a master cyl means a lot of air in the system, so they may need more bleeding.
This sounds like a vacuum or hydrolic issue. If The pedal is spongy soft while its running it indicates air in the brake line, or faulty master cylinder, or faulty brake booster. A faulty master cylinder would get hard after a few pumps of the pedal. Since the pedal is hard when it is shut down, it makes me lean toward the brake booster.
have someone have there foot on the brake & start vehicle, inspect the flexible brakelines to see if any are balooning out. You could also clamp off one flexible line at a time & press brake & when you find the brake at fault the pedal will go hard.
it sounds like there is still air in the brake system that has not been removed.you don`t say if you replaced the brake pads and shoes(in which case you shouldn`t need to bleed)or you replaced a brake line(s) or wheel cylinder or caliper.you will have to keep bleeding the brakes all 4 wheels(trucks don`t use bi-directional braking meaning right rear brakes with left front and so on).bleed both rear brakes then bleed both front brakes,was this problem there before you replaced the brakes(soft pedal noticed) and this is why you changed the brakes or the pedal was soft and the brakes needed changing.its possible that the master cylinder has an internal leak and may need replacing.
try bleeding the master cylinder at the lines first as it indicates that there is still air in the pistons
loosen one line at a time and gently apply the brakes until any air bubbles come out tighten the line before releasing the brake pedal
do it for the other line and then bleed all the brakes starting at the longest line first and ending at the shortest line
check all cylinders, the MC is only one of the components that operates with brake fluid, you have brake calipers that can leak and also wheel cylinders on the rear of cars equipped with rear drum brakes.. check all of these for brake fluid seepage.. reapir as needed... also check for air in the system if the system has been opened recently or any recenmt service...
this problem is caused by a vacuum leak inside the power brake vacuum booster unit (rubber diaphram), here is a photo of that part which is under the hood, the brake master cylinder is mounted to it.
You need to bleed the rear brakes to remove all air so they will work. Start w/ the wheel the greatest distance from the master cylinder, check master cyl. level often, if you draw in air, you have to start all over again. Repeat 'til all air is removed from system.
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
You most likely have a problem w/ the brake fluid being contaminated. Have u ever had the brake fluid changed? Everytime u need brakes, u should bleed the brakes to remove the old fluid. This is something most people DO NOT do, and the old fluid ruins the calipers and brake cylinders. Look in the master cylinder at the fluid. Is it dark, or clear? When the fluid goes bad, it'll not be clear. Fluid also absorbs moisture which is terrible for the brake system. Make sure any bottles of brake fluid are closed tightly after opening. Moisture and contaminants will corrode the inside of the calipers and brake cylinders, and the pistons stick and bind, and will finally totally seize up. Flush the system starting at the bleeder that's the greatest distance away from the master cylinder, then next greatest distance, 3rd greatest, then closest. Bleed each wheel until fluid runs out clear, and then a few more times to be sure u flushed it all out. Check the master cylinder often, so u don't run it so low it draws in air, or u will have more bleeding to do to get the air out too. U might be able to dissassemble the caliper if it's very recently changed, clean it up, check for any crud and coat w/ clean fluid, reassemble. Good Luck, let me know how u make out. countrycurt0
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