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Re: 1999 GMC Sierra brake problem
When you change the master cylinder it is best to gravity bleed first, then proceed to the pedal technique.
Gravity bleeding is when you just open the valve and let gravity pull the fluid down.
Keep in mind you must "prime" the master cylinder first before installing it, did you do that?
Priming the master cylinder is where you put it in a vise gently, you then take apaptors and screw into the outlets, then take tubes from the adaptors back into the resoirvor. You fill the resorvoir up with fluid and take a 3/8 extension for a socket and push the piston in and out slowly until there are no more air bubbles.
After this procedure you gravity bleed the brakes, then go on to the pedal technique.
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there is still air in the system. To bleed the brakes bleed out the m/cyl first by loosing off the lines at the cylinder v/ This primes the master cylinder first. Then starting at the longest line first bleed out the air finishing at the shortest line
Section 06-06: Hydraulic Brake System
1992 Festiva Workshop Manual
GENERAL SERVICE OPERATIONS
Read Hydraulic System Bleeding General Information.
Clean all dirt from the master cylinder filler cap.
Fill the master cylinder with the specified brake fluid.
During the bleeding operation do not allow the master cylinder to run dry.
If the master cylinder is known or suspected to contain air it must be bled before the wheel cylinders or calipers.
To bleed the master cylinder, loosen the front line fitting and have an assistant push the brake pedal slowly through its full travel. While the assistant holds the pedal, tighten the brake line fitting. After the line fitting is tightened, the assistant may release the brake pedal. Repeat this procedure on the rear brake line. Repeat the entire process several times to ensure all air has been removed from the master cylinder.
Remove the bleeder screw cap from the appropriate rear wheel cylinder.
Position a box end wrench on the bleeder fitting.
Attach a rubber hose to the bleeder fitting.
The hose has to fit snugly around the bleeder fitting.
Submerge the free end of the hose in a container partially filled with brake fluid.
Loosen the bleeder fitting approximately three quarters of a turn.
Have an assistant push the brake pedal slowly through its full travel and hold it there.
Close the bleeder fitting.
Have the assistant release the brake pedal.
Repeat Steps 9 through 12 until air bubbles cease to appear at the submerged end of the bleeder hose.
When the fluid entering the bottle is completely free of bubbles, tighten the bleeder screw, remove the bleeder hose, and install the bleeder screw cap.
Repeat Steps 5 through 14 at the appropriate diagonal front caliper.
Check the master cylinder fluid level.
If necessary fill it to the correct level with the specified brake fluid.
Check pedal feel. If the pedal remains spongy, repeat the bleeding process or, if necessary, refer to Diagnosis and Testing in Section 06-00.
The brake system bleeding procedure differs for ABS and non-ABS
vehicles. The following procedure pertains only to non-ABS vehicles. For
details on bleeding ABS equipped vehicles, refer to the ABS procedures
later in this section.
Make sure the master cylinder contains clean DOT 3 brake fluid at all times during the procedure.
The master cylinder must be bled first if it is suspected of containing air. Bleed the master cylinder as follows:
Position a container under the master cylinder to catch the brake fluid.
Loosen the left front brake line (front upper port) at the master cylinder and allow the fluid to flow from the front port.
Connect the line and tighten to 24 ft. lbs. (32 Nm).
Have an assistant depress the brake pedal slowly one time and hold
it down, while you loosen the front line to expel air from the master
cylinder. Tighten the line, then release the brake pedal. Repeat until
all air is removed from the master cylinder.
Tighten the brake line to 24 ft. lbs. (32 Nm) when finished.
Repeat these steps for the right front brake line (rear upper port) at the master cylinder.
Do not allow brake fluid to spill on or come in contact with the
vehicle' finish, as it will remove the paint. In case of a spill,
immediately flush the area with water.
If a single line or fitting was the only hydraulic line
disconnected, then only the caliper(s) or wheel cylinder(s) affected by
that line must be bled. If the master cylinder required bleeding, then
all calipers and wheel cylinders must be bled in the proper sequence:
Bleed the individual calipers or wheel cylinders as follows:
Place a suitable wrench over the bleeder screw and attach a clear plastic hose over the screw end.
Submerge the other end in a transparent container of brake fluid.
Loosen the bleed screw, then have an assistant apply the brake
pedal slowly and hold it down. Close the bleed screw, then release the
brake pedal. Repeat the sequence until all air is expelled from the
caliper or cylinder.
When finished, tighten the bleed screw to 97 inch lbs. (11 Nm) for the front, or 66 inch lbs. (7.5 Nm) for the rear.
Check the pedal for a hard feeling with the engine not running. If
the pedal is soft, repeat the bleeding procedure until a firm pedal is
Fig. 1: Loosen the front brake line in order to bleed the master cylinder
Fig. 2: Connect a bleed hose from the bleed valve on the front caliper to a jar of brake fluid
Fig. 3: Always follow the lettered sequence when bleeding the hydraulic brake system
Hope this helps to solve it; remember to rate this answer.
It's very possible to be the master cylinder, the o-rings on the piston could be worn, letting the fluid seep back through instead of getting full pressure through the lines. Could also be the brake booster.
Try changing the master cylinder first, which is cheaper and easier, and see if there is a difference, you'll need to bleed the brakes again.
To check the booster,
With the engine off, pump the brakes until the pedal is hard, then hold pressure on the brake pedal and listen for air escaping, or the pedal goes slowly to the floor, Do that after the master cylinder has been changed, at least that will be eliminated.
When you fixed the brakes, did you take the lid off the master cylinder. I just wonder if you spilled out your brake fluid. The only reason the pedal should go to the floor is if the brake fluid is gone or the master cylinder is not working. Check the fuse. Check the brake fluid level. The pedal only goes to the floor if there is no back pressure in the lines between the master cylinder the lines to the brake pistion on the calipers. Please give me a good ratin and keep me informed.
loosen the front brake line that goes to the master cylinder fill master cylinder up with fluid and bleed it first by attaching a flexible line from master cylinder front brake side submerge that line into jar of brakefluid keeping air from entering line get someone to pump brake pedal while ur holding line in a jar keep refilling the brake resivoir so no air gets in, once the master is bled front brake side then reattach the origional brake line then remove bleed screw from caliper and let fluid gravity flow down to calliper then bleed the normal way. more than likely your master cylinder has air trapped in the front portion good luck