Check the inside of the back wheels and see if you don’t have a leak at the brake line input to the cylinders. And you probably have a leak somewhere.
I am assuming you bench bled the master? I had a Honda with same problem. I put a tube connected to loosened bleeder valve on rear wheels in a jar of fluid and got someone to pump brakes while I watched...took a while but eventually it put out a bunch of sludge and tons of air...then tighted the valve and brakes pumped up rock hard. Try that.
As you have replaced the brake booster and master cylinder I strongly suggest you begin by rechecking the fitment to ensure any pedal linkage adjustment is correct and a clearance exists between the booster pushrod and the master cylinder piston.
Ensure the load sensing valve is correctly adjusted and is working correctly.
Check the brake at each wheel is in the condition the manufacturer would approve of and then bleed all four brakes using the most suitable method...
SOURCE: 2002 Toyota Sequoia power brake booster % master cylinder are bad
Mr. Clark:
It would be extraordinary for a technician to recommend a booster and a master cylinder with no previous mention from you of a possible problem in this area. In other words, the technician had no business looking at the m/cyl or the booster, unless you told him that your braking was deficient.
Here is what REALLY happened: Somewhere, I'd bet, your brake light came on, and somebody "topped off" the master cylinder, to shut the light off. Now, please understand that the brake fluid level in the master cylinder reservoir DROPS PROPORTIONALLY to the wear of the brake pads. When the tech replaced the front brake pads, he obviously had to squeeze the caliper pistons into the housings to allow the insertion of the new pads. This action forced the brake fluid from the now-compressed caliper pistons BACK into the master cylinder, and it sprayed out from under the cap, coating the cylinder and the booster. Tell that repair service that you do NOT need a booster and a master cylinder, THEY caused the problem with the fluid being everywhere. I am confident that I just saved you a significent amount of money that you do NOT need to spend. Have a great day.
SOURCE: 1998 chevy s10- brake pedal goes to the floor-
Did you put fresh fluid in the master cylinder? I really think you still have air in the lines. To bleed manually, start with the bleeder the greatest distance from the master cylinder, car running, helper pushing down about half way on brake pedal and releasing 3 times, on 3rd time, holding pedal down to half way depressed point while you open the bleeder valve. Repeat until you are sure all air has been flushed out. Check master cyl reservoir level often, because if it gets low and you **** air into the system, you have to start all over again. Go to wheel next greatest distance from master cyl, repeat above, working your way to wheel closest to master cyl last.
SOURCE: SOFT BRAKE PEDAL ON MY DAEWOO
MAKE SURE THE ARE NO OTHER LEAKS.IF U DID NOT BENCH BLEED THE MASTER CYLINDER BEFORE U PUT IT ON U PROBLEBLY HAVE AIR IN THE SYSTEM.BLEED THE MASTER CYLINDER AT THE LINES THAT COME OFF OF IT.THEN U NEED TO BLEED THE REAT OF THE BRAKES IN THIS ORDER.RIGHT REAR,LEFT FRONT,LEFT REAR.RIGHT FRONT.WHEN BLEEDING THE BRAKES WAIT 15 SECONDS BETWEEN PUMPS OF THE BRAKE PEDAL.IF U GO TO FAST U CAN DAMAGE THE MASTER CYLINDER.GOOD LUCK
SOURCE: Brake problem
do it again .. bleed the master cylinder maybe more than once .. I have had this happen before they are tough sometimes to get to take fluid right .. bleed it again till fluid is really flowing right .. your problem is the master cylinder not blead enough yet .. holler if you have more Jerry
SOURCE: brake pedal goes soft when engin is running on 1998 chevy 1500
If your rubber brake lines are from the factory those hoses can stretch when the brakes are applied. This same thing happened to me. I replaced the master cylinder, with little to no change. So then I changed the brake booster. And if you read the fine print if either the brake booster or the master cylinder go bad it can take the other with it. So if the rubber brake lines don't fix this I would look at replacing both the master cylinder and the brake booster at the same time. But if you would like to trouble shoot this issue a bit more you can clamp off just the rear brakes or just the front brakes or even clamp off 3 of 4 or if it might be a issue before it even hits the brakes them self you can clamp off all 4 of the brakes to see were the problem is. This is to test fewer components at once and to see if the issue is with the cylinders / calipers or hoses. This will help you determine the location of the problem faster and with out throwing parts at your truck and hopping it fixes the issue.
Fig. Fig 1: Common power brake booster attachment
Fig. Fig. 2: Checking the piston booster rod using the gauge tool
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